Nanowires-based transparent conductors

ABSTRACT

A method for forming a transparent conductor including a conductive layer coated on a substrate is described. The method comprises depositing a plurality of metal nanowires on a surface of a substrate, the metal nanowires being dispersed in a liquid; and forming a metal nanowire network layer on the substrate by allowing the liquid to dry.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/504,822 filed Aug. 14, 2006, now pending; which claims the benefitunder 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No.60/707,675 filed Aug. 12, 2005, U.S. Provisional Patent Application No.60/796,027 filed Apr. 28, 2006, and U.S. Provisional Patent ApplicationNo. 60/798,878 filed May 8, 2006; all of these applications areincorporated herein by reference in their entireties.

BACKGROUND

Technical Field

This invention is related to transparent conductors and methods ofmanufacturing the same, in particular, to high-throughput coatingmethods.

Description of the Related Art

Transparent conductors refer to thin conductive films coated onhigh-transmittance insulating surfaces or substrates. Transparentconductors may be manufactured to have surface conductivity whilemaintaining reasonable optical transparency. Such surface conductingtransparent conductors are widely used as transparent electrodes in flatliquid crystal displays, touch panels, electroluminescent devices, andthin film photovoltaic cells, as anti-static layers and aselectromagnetic wave shielding layers.

Currently, vacuum deposited metal oxides, such as indium tin oxide(ITO), are the industry standard materials to provide opticallytransparency and electrical conductivity to dielectric surfaces such asglass and polymeric films. However, metal oxide films are fragile andprone to damage during bending or other physical stresses. They alsorequire elevated deposition temperatures and/or high annealingtemperatures to achieve high conductivity levels. There also may beissues with the adhesion of metal oxide films to substrates that areprone to adsorbing moisture such as plastic and organic substrates, e.g.polycarbonates. Applications of metal oxide films on flexible substratesare therefore severely limited. In addition, vacuum deposition is acostly process and requires specialized equipment. Moreover, the processof vacuum deposition is not conducive to forming patterns and circuits.This typically results in the need for expensive patterning processessuch as photolithography.

Conductive polymers have also been used as optically transparentelectrical conductors. However, they generally have lower conductivityvalues and higher optical absorption (particularly at visiblewavelengths) compared to the metal oxide films, and suffer from lack ofchemical and long-term stability.

Accordingly, there remains a need in the art to provide transparentconductors having desirable electrical, optical and mechanicalproperties, in particular, transparent conductors that are adaptable toany substrates, and can be manufactured and patterned in a low-cost,high-throughput process.

BRIEF SUMMARY

In one embodiment, it is described herein a transparent conductorcomprising: a substrate; and a conductive layer on the substrate, theconductive layer including a plurality of nanowires, preferably, metalnanowires.

In another embodiment, a transparent conductor comprises a substrate;and a conductive layer on the substrate, the conductive layer includinga plurality of metal nanowires embedded in a matrix, in particular, anoptically clear polymeric matrix.

In yet another embodiment, the transparent conductor further comprises acorrosion inhibitor.

In a further embodiment, it is described herein a method of fabricatinga transparent conductor comprising: depositing a plurality of metalnanowires on a substrate, the metal nanowires being dispersed in afluid; and forming a metal nanowire network layer on the substrate byallowing the liquid to dry.

In another embodiment, a method comprises depositing a plurality ofmetal nanowires on a substrate, the metal nanowires being dispersed in afluid; and forming a metal nanowire network layer on the substrate byallowing the liquid to dry; depositing a matrix material on the metalnanowire network layer; and curing the matrix material to form a matrix,the matrix and the metal nanowires embedded therein forming a conductivelayer.

In a further embodiment, the method described herein can be performed ina reel-to reel process, wherein the substrate is driven by a rotatingreel along a traveling path, and the depositing of the metal nanowiresis carried out at a first deposition station along the traveling path,and the depositing of the matrix material is carried out at a seconddeposition station along the traveling path.

In another embodiment, the conductive layer can be patterned, inparticular, photo-patterned by using photo-curable matrix materials.

In another embodiment, it is described herein a laminated structurecomprising: a flexible donor substrate; and a conductive layer includinga matrix embedded with a plurality of metal nanowires.

In a further embodiment, a laminating process is described, the processcomprising: applying the laminated structure to a substrate of choice,and removing the flexible donor substrate.

In yet another embodiment, a display device is described, the displaydevice comprising at least one transparent electrode having a conductivelayer, the conductive layer including a plurality of metal nanowires. Inparticular, the conductive layer comprises the metal nanowires in anoptically clear polymeric matrix.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not drawn to scale, and some of these elementsare arbitrarily enlarged and positioned to improve drawing legibility.Further, the particular shapes of the elements as drawn are not intendedto convey any information regarding the actual shape of the particularelements, and have been selected solely for ease of recognition in thedrawings.

FIG. 1 is a schematic illustration of a nanowire.

FIG. 2 is a graph illustrating the expected optical properties of asilver nanoellipsoids at various wavelengths of light.

FIG. 3 illustrates the absorption spectrum of a silver nanowire layer ona polyethylene terephthalate (PET) substrate.

FIG. 4 is a graph illustrating expected values for various resistivityproperties of a nanowire based on the wire diameter.

FIG. 5 is a graph illustrating the expected overall resistivity as afunction of the diameters of nanowires.

FIG. 6 shows an SEM image of a single silver nanowires connectingbetween two metal contacts.

FIG. 7 illustrates a network of filamentous proteins that function asbiological templates for a transparent conductor.

FIG. 8 illustrates a protein scaffold coupled to conductive particlesvia various binding sites.

FIG. 9 illustrates the formation of a conductive network of biologicaltemplates based on the coupling of associate peptides.

FIG. 10A illustrates schematically an embodiment of a metalnanowires-based transparent conductor.

FIG. 10B illustrates schematically another embodiment of a metalnanowires-based transparent conductor.

FIG. 10C shows schematically a further embodiment of a metal nanowirebased transparent conductor in which portions of the nanowires areexposed on a surface of the transparent conductor.

FIG. 10D shows an SEM image of silver nanowires protruding out of asurface of the transparent conductor.

FIG. 10E illustrates schematically another embodiment of a metalnanowires-based transparent conductor.

FIG. 11 illustrates schematically a further embodiment of a metalnanowires-based transparent conductor having a multi-layer structure.

FIG. 12 shows a transparent conductor structure having a reservoir fordelivering a vapor phase inhibitor (VPI).

FIGS. 13A-13D show an example of a fabrication process of a transparentconductor.

FIG. 14A shows an example of a fabrication process of a transparentconductor by web coating.

FIG. 14B shows another example of a fabrication process of a transparentconductor by web coating.

FIG. 15A shows a web coating system and a flow process for fabricating atransparent conductor.

FIG. 15B shows an SEM image of a conductive layer following apost-treatment of pressure application.

FIGS. 16A-16C show an example of a lamination process.

FIGS. 17A-17C show another example of a lamination process.

FIG. 18 shows an example of photo-patterning a conductive layer.

FIGS. 19A-19B show an example of a continuous photo-patterning methodsuitable for a web coating process.

FIG. 20 shows a partial system and a process of fabricating a patternedtransparent conductor.

FIG. 21 shows a display device comprising transparent electrodes basedon metal nanowires.

FIG. 22 shows a touch screen device comprising two transparentconductors based on metal nanowires.

FIG. 23 shows a typical release profile of H₂S gas from freshly cookedegg yolks.

FIG. 24A shows the light transmissions of six samples of conductivefilms before and after an accelerated H₂S corrosion test.

FIG. 24B shows the resistances of six samples of conductive films beforeand after an accelerated H₂S corrosion test.

FIG. 24C shows the hazes of six samples of conductive films before andafter an accelerated H₂S corrosion test.

FIG. 25A shows an example of directly patterning a nanowire-basedtransparent conductive film.

FIG. 25B shows photographs of the patterned conductive films before andafter an adhesive tape treatment.

FIGS. 26A-26F show photographs of the patterned conductive films beforeand after an adhesive tape treatment at various levels of magnification.

FIGS. 27A-27D show photographs of another exemplary conductive filmbefore and after a solvent treatment.

DETAILED DESCRIPTION

Certain embodiments are directed to a transparent conductor based on aconductive layer of nanowires. In particular, the conductive layerincludes a sparse network of metal nanowires. In addition, theconductive layer is transparent, flexible and can include at least onesurface that is conductive. It can be coated or laminated on a varietyof substrates, including flexible and rigid substrates. The conductivelayer can also form part of a composite structure including a matrixmaterial and the nanowires. The matrix material can typically impartcertain chemical, mechanical and optical properties to the compositestructure. Other embodiments describe methods of fabricating andpatterning the conductive layer.

Conductive Nanowires

FIG. 1 illustrates a nanowire 2 having an aspect ratio equal to thelength L₁ divided by the diameter d₁. Suitable nanowires typically haveaspect ratios in the range of 10 to 100,000. Larger aspect ratios can befavored for obtaining a transparent conductor layer since they mayenable more efficient conductive networks to be formed while permittinglower overall density of wires for a high transparency. In other words,when conductive nanowires with high aspect ratios are used, the densityof the nanowires that achieves a conductive network can be low enoughthat the conductive network is substantially transparent.

One method to define the transparency of a layer to light is by itsabsorption coefficient. The illumination of light passing through alayer can be defined as:I=I_(o)e^(−ax)in which I_(o) is the incoming light on a first side of the layer, I isthe illumination level that is present on a second side of the layer,and e^(−ax) is the transparency factor. In the transparency factor, a isthe absorption coefficient and x is the thickness of the layer. A layerhaving a transparency factor near 1, but less than 1 can be consideredto be substantially transparent.

FIGS. 2-5 illustrate some of the optical and electrical characteristicsof the conductive nanowires.

FIG. 2 shows a theoretical model of the light absorption of silvernanoellipsoids at various wavelengths of light. Depending on widths andlengths, silver nanoellipsoids exhibit a high extinction coefficient toa narrow band of light in the wavelengths between 400 and 440 nanometersand to wavelengths of light above 700 nm. However, they aresubstantially transparent between about 440 to about 700 nm, which fallsin the visible range.

FIG. 3 shows the absorption spectrum of a layer of silver nanowiresdeposited on a polyethylene terephthalate (PET) substrate. As shown bythe absorption profile, the silver nanowire layer on PET substrate issubstantially transparent between about 440 nm to 700 nm, agreeing withthe results of the theoretical model shown in FIG. 2.

FIGS. 4 and 5 show the results of theoretical modeling of theresistivity of metal nanowires based on their diameters. For a largerdiameter of nanowire, the resistivity decreases substantially althoughit will absorb more light. As can be seen in FIG. 4, the effects onresistivity based on the grain boundary and surface scattering are highat diameters of less than 10 nm. As the diameter increases, theseeffects are drastically reduced. The overall resistivity is thereforereduced greatly for diameter that increases from 10 nm to over 100 nm(see, also FIG. 5). This improvement in electrical properties must bebalanced, however, against the decreased transparency for applicationsrequiring a transparent conductor.

FIG. 6 shows a single Ag nanowire 4 that extends between two otherelectrical terminals 6 a and 6 b, to provide an electrically conductivepath from terminal 6 a to terminal 6 b. The term “terminal” includescontact pads, conduction nodes and any other starting and ending pointsthat may be electrically connected. The aspect ratio, size, shape andthe distribution of the physical parameters of the nanowires areselected to provide the desired optical and electrical properties. Thenumber of such wires that will provide a given density of Ag nanowiresis selected to provide acceptable electrical conduction properties forcoupling terminal 6 a to terminal 6 b. For example, hundreds of Agnanowires 4 can extend from terminal 6 a to 6 b to provide a lowresistance electrical conduction path, and the concentration, aspectratio, size and shape can be selected to provide a substantiallytransparent conductor. Therefore, transparent, electrical conduction isprovided from terminal 6 a to terminal 6 b using a plurality of Agnanowires.

As can be appreciated, the distance from terminal 6 a to terminal 6 bmay be such that the desired optical properties are not obtained with asingle nanowire. A plurality of many nanowires may need to be linked toeach other at various points to provide a conductive path from terminal6 a to terminal 6 b. According to the invention, the nanowires areselected based on the desired optical properties. Then, the number ofnanowires that provides the desired conduction path and overallresistance on that path are selected to achieve acceptable electricalproperties for an electrical conduction layer from terminal 6 a toterminal 6 b.

The electrical conductivity of the transparent layer is mainlycontrolled by a) the conductivity of a single nanowire, b) the number ofnanowires between the terminals, and c) the connectivity between thenanowires. Below a certain nanowire concentration (also referred as thepercolation threshold), the conductivity between the terminals is zero,i.e. there is no continuous current path provided because the nanowiresare spaced too far apart. Above this concentration, there is at leastone current path available. As more current paths are provided, theoverall resistance of the layer will decrease.

Conductive nanowires include metal nanowires and other conductiveparticles having high aspect ratios (e.g., higher than 10). Examples ofnon-metallic nanowires include, but are not limited to, carbon nanotubes(CNTs), metal oxide nanowires, conductive polymer fibers and the like.

As used herein, “metal nanowire” refers to a metallic wire comprisingelement metal, metal alloys or metal compounds (including metal oxides).At least one cross sectional dimension of the metal nanowire is lessthan 500 nm, and less than 200 nm, and more preferably less than 100 nm.As noted above, the metal nanowire has an aspect ratio (length:width) ofgreater than 10, preferably greater than 50, and more preferably greaterthan 100. Suitable metal nanowires can be based on any metal, includingwithout limitation, silver, gold, copper, nickel, and gold-platedsilver.

The metal nanowires can be prepared by known methods in the art. Inparticular, silver nanowires can be synthesized through solution-phasereduction of a silver salt (e.g., silver nitrate) in the presence of apolyol (e.g., ethylene glycol) and poly(vinyl pyrrolidone). Large-scaleproduction of silver nanowires of uniform size can be prepared accordingto the methods described in, e.g., Xia, Y. et al., Chem. Mater. (2002),14, 4736-4745, and Xia, Y. et al., Nanoletters (2003) 3(7), 955-960.

Alternatively, the metal nanowires can be prepared using biologicaltemplates (or biological scaffolds) that can be mineralized. Forexample, biological materials such as viruses and phages can function astemplates to create metal nanowires. In certain embodiments, thebiological templates can be engineered to exhibit selective affinity fora particular type of material, such as a metal or a metal oxide. Moredetailed description of biofabrication of nanowires can be found in,e.g., Mao, C. B. et al., “Virus-Based Toolkit for the Directed Synthesisof Magnetic and Semiconducting Nanowires,” (2004) Science, 303, 213-217.Mao, C. B. et al., “Viral Assembly of Oriented Quantum Dot Nanowires,”(2003) PNAS, vol. 100, no. 12, 6946-6951; Mao, C. B. et al., “ViralAssembly of Oriented Quantum Dot Nanowires,” (2003) PNAS, 100(12),6946-6951, U.S. application Ser. No. 10/976,179, and U.S. ProvisionalApplication Ser. No. 60/680,491, which references are incorporatedherein in their entireties.

More specifically, a conductive material or a conductor (e.g., a metalnanowire) can directly bind to a biological template based on anaffinity between the conductive material and certain binding sites(e.g., peptide sequences) on the biological template.

In other embodiments, a conductive material can be created by anucleation process, during which a precursor is converted to conductiveparticles that bind to the biological templates, the conductiveparticles being capable of further growing into a continuous conductivelayer. This process is also referred to as “mineralization” or“plating”. For example, a metal precursor (e.g., a metal salt) can beconverted to an elemental metal in the presence of a reducing agent. Theresulting elemental metal binds to the biological templates and growsinto a continuous metallic layer.

In other embodiments, a seed material layer is initially nucleated ontothe biological material. Thereafter, a metal precursor can be convertedinto metal and plated on the seed material layer. The seed material canbe selected, for example, based on a material that causes the nucleationand growth of a metal out of a solution containing a corresponding metalprecursor. To illustrate, a seed material layer containing palladium cancause the mineralization of Cu or Au. As one specific example, forcreating a Cu conductor, acceptable seed materials may containpalladium, a palladium based molecule, Au or an Au based molecule. Foran oxide conductor, a zinc oxide may be used as a nucleation material.Examples of the seed material include Ni, Cu, Pd, Co, Pt, Ru, Ag, Coalloys or Ni alloys. Metals, metal alloys and metal oxides that can beplated include, without limitation, Cu, Au, Ag, Ni, Pd, Co, Pt, Ru, W,Cr, Mo, Ag, Co alloys (e.g., CoPt), Ni alloys, Fe alloys (e.g., FePt) orTiO₂, Co₃O₄, Cu₂O, HfO₂, ZnO, vanadium oxides, indium oxide, aluminumoxide, indium tin oxide, nickel oxide, copper oxide, tin oxide, tantalumoxide, niobium oxide, vanadium oxide or zirconium oxide.

Any of a number of different biological materials can be used to providethe templates for creating the metal nanowires, including proteins,peptides, phages, bacteria, viruses, and the like. The techniques forselecting, forming and engineering a biological material that willcouple to a desired metal or conductive material are described in U.S.application Ser. Nos. 10/155,883 and 10/158,596; both applications arein the name of Cambrios Technologies Corporation and are incorporatedherein by reference.

As noted above, biological templates such as protein, a peptide, orother biological material can be engineered to have affinity sites for aselected seed material or a selected conductive material. Proteins orpeptides with affinity to a specific material can be identified througha protein discovery process such as phage display, yeast display, cellsurface display or others. For example in the case of phage display,libraries of phages (e.g., M13 phages) can be created by inserting awide variety of different sequences of peptides into a population of thephage. A protein having high affinity for a specific target molecule canbe isolated and its peptide structure can be identified.

In particular, the genetic sequences of the biological molecules can becontrolled to provide a number of copies of particular peptide sequencesin certain types of phage particles. For example, about 3000 copies ofP8 proteins can be arranged in an ordered array along the length of M13phage particles. The P8 proteins can be modified to include a specificpeptide sequence that can nucleate the formation of or bind a conductivematerial, thereby providing conductive nanowires of high conductivity.Advantageously, this technique allows for the ability to control thegeometry and crystalline structure of the nanowires through the use ofbiological template molecules, e.g., proteins having specificallydesigned or controlled peptide sequences. To that end, peptides orproteins with binding affinity for silver, gold or palladium have beenidentified which can be incorporated into a phage structure to createnanowires with dimensions based on those of the phage particles.

Biological materials other than phages can be used as templates for theformation of conductive nanowires. For example, filamentous proteinswhich self-assemble into long strands of tens of microns in length canbe used as an alternative template (see, FIG. 7). Advantageously, such atemplate protein can be synthesized to have a much larger aspect ratiothan phage, which leads to lower percolative threshold concentrations ofthe conductive nanowires. Additionally, proteins are easier tosynthesize in large volume than phage particles. Large scale manufactureof proteins, such as enzymes used as detergent additives, is welldeveloped.

FIG. 8 shows a schematic version of a protein scaffold 8 having a numberof binding sites 8 a coupled with conductive particles 8 b. The bindingsites are selected to have an affinity for the conductive particles,such as Au, Ag, Cu and Ni. Alternatively, the binding sites 8 a have anaffinity for a seed material layer (e.g., Pd and Au) that can furthernucleate the conductive particles, such as Cu and the like. The proteinscaffold 8 can also be engineered to have a plurality of binding sites 8a with such affinity. It is preferred to have them spaced at frequentand regular intervals along their length to increase the conductivity ofthe final conductive layer.

The length of a biological material, such as a protein, as well as itsdiameter is easily engineered using known techniques. It is engineeredto have the correct dimensions for the optical properties. Once thesize, shape and aspect ratio have been selected, the biological materialcan be exposed to conductive material 8 b, such as a metal, or aprecursor of the metal.

FIG. 9 illustrates a further embodiment of fabricating conductivenanowires using biological templates. The protein scaffold 8 can befurther engineered to include binding partners such as associatepeptides 9 a and 9 b at respective ends. Binding partners can couplewith each other through any type of associative interaction, including,for example, ionic interaction, covalent bonding, hydrogen bonding,hydrophobic interaction, and the like. The interaction between theassociate peptides 9 a and 9 b encourage the self-assembly of theconductive nanowires into 2-D interconnected mesh networks, as shown inthe final sequence in FIG. 8. The association peptides and theirlocations may be of the type to encourage the forming of meshes, end toend connection, cross-connections, and other desired shapes for theconductive layer. In the example shown in FIG. 8, the conductivematerial 8 b has already bound to the protein scaffold 8 before theprotein scaffolds form a network. It should be understood, that proteinscaffold 8 can also form a network prior to the binding of theconductive material.

Thus the use of biological template having associate peptides or otherbinding partners allows for the formation of a conductive layer ofhighly connected network than would be possible with random nanowires.The particular network of the biological templates can therefore beselected to achieve a desired degree of order in the conductive layer.

Template-based synthesis is particularly suited for fabricatingnanowires of particular dimensions, morphologies and compositions.Further advantages of biologically based manufacturing of nano-materialsinclude: solution processing that can be modified for high throughput,ambient temperature deposition, superior conformality and production ofconductive layers.

Conductive Layer and Substrate

As an illustrative example, FIG. 10A shows a transparent conductor 10comprising a conductive layer 12 coated on a substrate 14. Theconductive layer 12 comprises a plurality of metal nanowires 16. Themetal nanowires form a conductive network.

FIG. 10B shows another example of a transparent conductor 10′, in whicha conductive layer 12′ is formed on the substrate 14. The conductivelayer 12′ includes a plurality of metal nanowires 16 embedded in amatrix 18.

“Matrix” refers to a solid-state material into which the metal nanowiresare dispersed or embedded. Portions of the nanowires may protrude fromthe matrix material to enable access to the conductive network. Thematrix is a host for the metal nanowires and provides a physical form ofthe conductive layer. The matrix protects the metal nanowires fromadverse environmental factors, such as corrosion and abrasion. Inparticular, the matrix significantly lowers the permeability ofcorrosive elements in the environment, such as moisture, trace amount ofacids, oxygen, sulfur and the like.

In addition, the matrix offers favorable physical and mechanicalproperties to the conductive layer. For example, it can provide adhesionto the substrate. Furthermore, unlike metal oxide films, polymeric ororganic matrices embedded with metal nanowires can be robust andflexible. As will be discussed in more detail herein, flexible matricesmake it possible to fabricate transparent conductors in a low-cost, highthroughput process.

Moreover, the optical properties of the conductive layer can be tailoredby selecting an appropriate matrix material. For example, reflectionloss and unwanted glare can be effectively reduced by using a matrix ofa desirable refractive index, composition and thickness.

Typically, the matrix is an optically clear material. A material isconsidered optically clear if the light transmission of the material isat least 80% in the visible region (400 nm-700 nm). Unless specifiedotherwise, all the layers (including the substrate) in a transparentconductor described herein are preferably optically clear. The opticalclarity of the matrix is typically determined by a multitude of factors,including without limitation: the refractive index (RI), thickness,consistency of RI throughout the thickness, surface (includinginterface) reflection, and haze (a scattering loss caused by surfaceroughness and/or embedded particles).

In certain embodiments, the matrix is about 10 nm to 5 μm thick, about20 nm to 1 μm thick, or about 50 nm to 200 nm thick. In otherembodiments, the matrix has a refractive index of about 1.3 to 2.5, orabout 1.35 to 1.8.

In certain embodiments, the matrix is a polymer, which is also referredto as a polymeric matrix. Optically clear polymers are known in the art.Examples of suitable polymeric matrices include, but are not limited to:polyacrylics such as polymethacrylates (e.g., poly(methylmethacrylate)), polyacrylates and polyacrylonitriles, polyvinylalcohols, polyesters (e.g., polyethylene terephthalate (PET), polyesternaphthalate, and polycarbonates), polymers with a high degree ofaromaticity such as phenolics or cresol-formaldehyde (Novolacs®),polystyrenes, polyvinyltoluene, polyvinylxylene, polyimides, polyamides,polyamideimides, polyetherimides, polysulfides, polysulfones,polyphenylenes, and polyphenyl ethers, polyurethane (PU), epoxy,polyolefins (e.g. polypropylene, polymethylpentene, and cyclic olefins),acrylonitrile-butadiene-styrene copolymer (ABS), cellulosics, siliconesand other silicon-containing polymers (e.g. polysilsesquioxanes andpolysilanes), polyvinylchloride (PVC), polyacetates, polynorbornenes,synthetic rubbers (e.g. EPR, SBR, EPDM), and fluoropolymers (e.g.,polyvinylidene fluoride, polytetrafluoroethylene (TFE) orpolyhexafluoropropylene), copolymers of fluoro-olefin and hydrocarbonolefin (e.g., Lumiflon®), and amorphous fluorocarbon polymers orcopolymers (e.g., CYTOP® by Asahi Glass Co., or Teflon® AF by Du Pont).

In other embodiments, the matrix is an inorganic material. For example,a sol-gel matrix based on silica, mullite, alumina, SiC, MgO—Al₂O₃—SiO₂,Al2O₃—SiO₂, MgO—Al₂O₃—SiO₂—Li₂O or a mixture thereof can be used.

In certain embodiments, the matrix itself is conductive. For example,the matrix can be a conductive polymer. Conductive polymers are wellknown in the art, including without limitation:poly(3,4-ethylenedioxythiophene) (PEDOT), polyanilines, polythiophenes,and polydiacetylenes.

“Conductive layer”, or “conductive film”, refers to a network layer ofmetal nanowires that provide the conductive media of the transparentconductor. When a matrix is present, the combination of the networklayer of metal nanowires and the matrix is also referred to as a“conductive layer”. Since conductivity is achieved by electrical chargepercolation from one metal nanowire to another, sufficient metalnanowires must be present in the conductive layer to reach an electricalpercolation threshold and become conductive. The surface conductivity ofthe conductive layer is inversely proportional to its surfaceresistivity, sometimes referred to as sheet resistance, which can bemeasured by known methods in the art.

Likewise, when a matrix is present, the matrix must be filled withsufficient metal nanowires to become conductive. As used herein,“threshold loading level” refers to a percentage of the metal nanowiresby weight after loading of the conductive layer at which the conductivelayer has a surface resistivity of no more than about 10⁶Ω/square (orΩ/□). The threshold loading level depends on factors such as the aspectratio, the degree of alignment, degree of agglomeration and theresistivity of the metal nanowires.

As is understood by one skilled in the art, the mechanical and opticalproperties of the matrix are likely to be altered or compromised by ahigh loading of any particles therein. Advantageously, the high aspectratios of the metal nanowires allow for the formation of a conductivenetwork through the matrix at a threshold surface loading levelpreferably of about 0.05 μg/cm² to about 10 μg/cm², more preferably fromabout 0.1 μg/cm² to about 5 μg/cm² and more preferably from about 0.8μg/cm² to about 3 μg/cm² for silver nanowires. These surface loadinglevels do not affect the mechanical or optical properties of the matrix.These values depend strongly on the dimensions and spatial dispersion ofthe nanowires. Advantageously, transparent conductors of tunableelectrical conductivity (or surface resistivity) and opticaltransparency can be provided by adjusting the loading levels of themetal nanowires.

In certain embodiments, the conductive layer spans the entire thicknessof the matrix, as shown in FIG. 10B. Advantageously, a certain portionof the metal nanowires is exposed on a surface 19 of the matrix due tothe surface tension of the matrix material (e.g., polymers). Thisfeature is particularly useful for touch screen applications. Inparticular, a transparent conductor can display surface conductivity onat least one surface thereof. FIG. 10C illustrates how it is believedthe network of metal nanowires embedded in a matrix achieves surfaceconductivity. As shown, while some nanowires, such as nanowire 16 a, maybe entirely ‘submerged’ in the matrix 18, ends of other nanowires, suchas end 16 b, protrude above the surface 19 of the matrix 18. Also, aportion of a middle section of nanowires, such as middle section 16 c,may protrude above the surface 19 of the matrix 18. If enough nanowireends 16 b and middle sections 16 c protrude above the matrix 18, thesurface of the transparent conductor becomes conductive. FIG. 10D is ascanning electron micrograph of the surface of one embodiment of atransparent conductor showing a contour of ends and middle sections ofnanowires protruding above a matrix in a transparent conductor.

In other embodiments, the conductive layer is formed by the metalnanowires embedded in a portion of the matrix, as shown in FIG. 10E. Theconductive layer 12″ occupies only a portion of the matrix 18 and arecompletely “submerged” in the matrix 18.

“Substrate”, or “substrate of choice”, refers to a material onto whichthe conductive layer is coated or laminated. The substrate can be rigidor flexible. The substrate can be clear or opaque. The term “substrateof choice” is typically used in connection with a lamination process, aswill be discussed herein. Suitable rigid substrates include, forexample, glass, polycarbonates, acrylics, and the like. Suitableflexible substrates include, but are not limited to: polyesters (e.g.,polyethylene terephthalate (PET), polyester naphthalate, andpolycarbonate), polyolefins (e.g., linear, branched, and cyclicpolyolefins), polyvinyls (e.g., polyvinyl chloride, polyvinylidenechloride, polyvinyl acetals, polystyrene, polyacrylates, and the like),cellulose ester bases (e.g., cellulose triacetate, cellulose acetate),polysulphones such as polyethersulphone, polyimides, silicones and otherconventional polymeric films. Additional examples of suitable substratescan be found in, e.g., U.S. Pat. No. 6,975,067.

Typically, the optical transparence or clarity of the conductive layercan be quantitatively defined by parameters including light transmissionand haze. “Light transmission” refers to the percentage of an incidentlight transmitted through a medium. In various embodiments, the lighttransmission of the conductive layer is at least 80% and can be as highas 98%. For a transparent conductor in which the conductive layer isdeposited or laminated on a substrate, the light transmission of theoverall structure may be slightly diminished. Performance-enhancinglayers, such as an adhesive layer, anti-reflective layer, anti-glarelayer, may further contribute to reducing the overall light transmissionof the transparent conductor. In various embodiments, the lighttransmission of the transparent conductors can be at least 50%, at least60%, at least 70%, or at least 80% and may be as high as at least 91% to92%.

Haze is an index of light diffusion. It refers to the percentage of thequantity of light separated from the incident light and scattered duringtransmission. Unlike light transmission, which is largely a property ofthe medium, haze is often a production concern and is typically causedby surface roughness and embedded particles or compositionalheterogeneities in the medium. In various embodiments, the haze of thetransparent conductor is no more than 10%, no more than 8%, or no morethan 5% and may be as low as no more than 2% to 0.5%.

Performance-enhancing Layers

As noted above, the conductive layers have superior physical andmechanical characteristics due to the matrix. These characteristics canbe further enhanced by introducing additional layers in the transparentconductor structure. Thus, in other embodiments, a multi-layertransparent conductor is described, which comprises one or more layerssuch as anti-reflective layers, anti-glare layers, adhesive layers,barrier layers, and hard coats.

As an illustrative example, FIG. 11 shows a multi-layer transparentconductor 20 comprising a conductive layer 12 and a substrate 14, asdescribed above. The multi-layer transparent conductor 20 furthercomprises a first layer 22 positioned over the conductive layer 12, asecond layer 24 positioned between the conductive layer 12 and thesubstrate 14, and a third layer 26 positioned below the substrate 14.Unless stated otherwise, each of the layers 22, 24 and 26 can be one ormore anti-reflective layers, anti-glare layers, adhesive layers, barrierlayers, hard coats, and protective films.

The layers 22, 24 and 26 serve various functions, such as enhancing theoverall optical performance and improving the mechanical properties ofthe transparent conductor. These additional layers, also referred to as“performance-enhancing layers”, can be one or more anti-reflectivelayers, anti-glare layers, adhesive layers, barrier layers, and hardcoats. In certain embodiments, one performance-enhancing layer providesmultiple benefits. For example, an anti-reflective layer can alsofunction as a hard coat and/or a barrier layer. In addition to theirspecific properties, the performance-enhancing layers are opticallyclear, as defined herein.

In one embodiment, layer 22 is an anti-reflective layer, layer 24 is anadhesive layer, and layer 26 is a hard coat.

In another embodiment, layer 22 is a hard coat, layer 24 is a barrierlayer, and layer 26 is an anti-reflective layer.

In yet another embodiment, layer 22 is a combination of ananti-reflective layer, anti-glare, a barrier layer and a hard coat,layer 24 is an adhesive layer, and layer 26 is an anti-reflective layer.

“Anti-reflective layer” refers to a layer that can reduce reflectionloss at a reflective surface of the transparent conductor. Theanti-reflective layer can therefore be positioned on the outer surfacesof the transparent conductor, or as an interface between layers.Materials suitable as anti-reflective layers are well known in the art,including without limitation: fluoropolymers, fluoropolymer blends orcopolymers, see, e.g., U.S. Pat. Nos. 5,198,267, 5,225,244, and7,033,729.

In other embodiments, reflection loss can be effectively reduced bycontrolling the thickness of the anti-reflective layer. For example,with reference to FIG. 11, the thickness of layer 22 can be controlledsuch that the light reflection of surface 28 and surface 30 cancel eachother out. Thus, in various embodiments, the anti-reflective layer isabout 100 nm thick or 200 nm thick.

Reflection loss can also be reduced by the appropriate use of texturedsurfaces, see, e.g. U.S. Pat. No. 5,820,957 and literature on AutoflexMARAG™ and Motheye™ products from MacDiarmid Autotype.

“Anti-glare layer” refers to a layer that reduces unwanted reflection atan outer surface of the transparent conductor by providing fineroughness on the surface to scatter the reflection. Suitable anti-glarematerials are well known in the art, including without limitation,siloxanes, polystyrene/PMMA blend, lacquer (e.g., butylacetate/nitrocellulose/wax/alkyd resin), polythiophenes, polypyrroles,polyurethane, nitrocellulose, and acrylates, all of which may comprise alight diffusing material such as colloidal or fumed silica. See, e.g.,U.S. Pat. Nos. 6,939,576, 5,750,054, 5,456,747, 5,415,815, and5,292,784. Blends and copolymers of these materials can have microscalecompositional heterogeneities, which can also exhibit light diffusionbehavior to reduce glare.

“Hard coat”, or “anti-abrasion layer” refers to a coating that providesadditional surface protection against scratches and abrasion. Examplesof suitable hard coats include synthetic polymers such as polyacrylics,epoxy, polyurethanes, polysilanes, silicones, poly(silico-acrylic) andso on. Typically, the hard coat also comprises colloidal silica. (See,e.g., U.S. Pat. Nos. 5,958,514, 7,014,918, 6,825,239, and referencescited therein.) The thickness of the hard coat is typically from about 1to 50 μm. The degree of hardness can be evaluated by known methods inthe art, such as by scratching the coating with a steel wool #000reciprocating 50 times within a distance of 2 cm at 2 reciprocations/secunder load of 300 g/cm² (see, e.g., U.S. Pat. No. 6,905,756). The hardcoat may be further exposed to an anti-glare process or ananti-reflection treatment by known methods in the art.

“Adhesive layer” refers to any optically clear material that bonds twoadjacent layers (e.g., conductive layer and substrate) together withoutaffecting the physical, electrical or optical properties of eitherlayer. Optically clear adhesive material are well known in the art,including without limitation: acrylic resins, chlorinated olefin resins,resins of vinyl chloride-vinyl acetate copolymer, maleic acid resins,chlorinated rubber resins, cyclorubber resins, polyamide resins,cumarone indene resins, resins of ethylene-vinyl acetate copolymer,polyester resins, urethane resins, styrene resins, polysiloxanes and thelike.

“Barrier layer” refers to a layer that reduces or prevents gas or fluidpermeation into the transparent conductor. It has been shown thatcorroded metal nanowires can cause a significant decrease in theelectrical conductivity as well as the light transmission of theconductive layer. The barrier layer can effectively inhibit atmosphericcorrosive gas from entering the conductive layer and contacting themetal nanowires in the matrix. The barrier layers are well known in theart, including without limitation: see, e.g. U.S. Patent Application No.2004/0253463, U.S. Pat. Nos. 5,560,998 and 4,927,689, EP Patent No.132,565, and JP Patent No. 57,061,025. Moreover, any of theanti-reflective layer, anti-glare layer and the hard coat can also actas a barrier layer.

In certain embodiments, the multi-layer transparent conductor mayfurther comprise a protective film above the conductive layer (e.g.,layer 22). The protective film is typically flexible and can be made ofthe same material as the flexible substrate. Examples of protective filminclude, but are not limited to: polyester, polyethylene terephthalate(PET), polybutylene terephthalate, polymethyl methacrylate (PMMA),acrylic resin, polycarbonate (PC), polystyrene, triacetate (TAC),polyvinyl alcohol, polyvinyl chloride, polyvinylidene chloride,polyethylene, ethylene-vinyl acetate copolymers, polyvinyl butyral,metal ion-crosslinked ethylene-methacrylic acid copolymers,polyurethane, cellophane, polyolefins or the like; particularlypreferable is PET, PC, PMMA, or TAC because of their high strength.

Corrosion Inhibitors

In other embodiments, the transparent conductor may comprise a corrosioninhibitor, in addition to, or in lieu of the barrier layer as describedabove. Different corrosion inhibitors may provide protection to themetal nanowires based on different mechanisms.

According to one mechanism, the corrosion inhibitor binds readily to themetal nanowires, forming a protective film on a metal surface. They arealso referred to as barrier-forming corrosion inhibitors.

In one embodiment, the barrier-forming corrosion inhibitor includescertain nitrogen-containing and sulfur-containing organic compounds,such as aromatic triazoles, imidazoles and thiazoles. These compoundshave been demonstrated to form stable complexes on a metal surface toprovide a barrier between the metal and its environment. For example,benzotriazole (BTA) is a common organic corrosion inhibitor for copperor copper alloys (Scheme 1). Alkyl substituted benzotriazoles, such astolytriazole and butyl benzyl triazole, can also be used. (See, e.g.,U.S. Pat. No. 5,270,364.) Additional suitable examples of corrosioninhibitors include, but are not limited to: 2-aminopyrimidine,5,6-dimethylbenzimidazole, 2-amino-5-mercapto-1,3,4-thiadiazole,2-mercaptopyrimidine, 2-mercaptobenzoxazole, 2-mercaptobenzothiazole,and 2-mercaptobenzimidazole.

Another class of barrier-forming corrosion inhibitors includesbiomolecules that show a particular affinity to the metal surface. Theseinclude small biomolecules, e.g. cysteine, and synthetic peptides andprotein scaffolds with fused peptide sequences with affinity for metals,e.g. EEEE; see, e.g. U.S. application Ser. Nos. 10/654,623,10/665,721,10/965,227,10/976,179, and 11/280,986, U.S. Provisional Application Ser.Nos. 60/680,491, 60/707,675 and 60/680,491.

Other barrier-forming corrosion inhibitors include dithiothiadiazole,alkyl dithiothiadiazoles and alkylthiols, alkyl being a saturated C₆-C₂₄straight hydrocarbon chain. This type of corrosion inhibitor canself-assemble on a metal surface to form a monolayer (Scheme 2), therebyprotecting the metal surface from corroding.

In a particular embodiment, the transparent conductor can comprise areservoir containing a corrosion inhibitor, providing a continuoussupply of the corrosion inhibitor in the vapor phase. The corrosioninhibitors suitable for such sustained delivery include “vapor phaseinhibitors” (VPI). VPIs are typically volatile solid materials thatsublime and form a monolayer on the surfaces of the metal nanowires.Advantageously, VPIs can be delivered to the metal surfaces andreplenished in a sustained manner for long-lasting protection. SuitableVPIs include barrier-forming inhibitors such as triazoles,dithiothiadiazole, alkyl dithiothiadiazoles and alkylthiols, asdescribed herein.

FIG. 12 illustrates such a transparent conductor structure suitable fora touch screen. More specifically, edge seals 32 and spacers 36 arepositioned between two conductive layers 12. In the space between thetwo conductive layers 12, one or more reservoirs 40 are present. Thereservoirs 40 are microscopic and are sparsely distributed such thattheir presence does not cause a reduction in the transmittance of thetransparent conductor. The reservoir contains a corrosion inhibitorwhich can be incorporated into a polymer matrix or impregnated into aporous material from which it can be sublimated into the vapor phase toform a monolayer 44 on the surface of the metal nanowires (see, inset).

According to another mechanism, a corrosion inhibitor binds more readilywith a corrosive element (e.g., H₂S) than with the metal nanowires.These corrosion inhibitors are known as “scavengers” or “getters”, whichcompete with the metal and sequester the corrosive elements. Examples ofH₂S scavengers include, but are not limited to: acrolein, glyoxal,triazine, and n-chlorosuccinimide. (See, e.g., Published U.S.Application No. 2006/0006120.)

In certain embodiments, the corrosion inhibitor (e.g., H₂S scavengers)can be dispersed in the matrix provided its presence does not adverselyaffect the optical or electrical properties of the conductive layer.

In other embodiments, the metal nanowires can be pretreated with acorrosion inhibitor before or after being deposited on the substrate.For example, the metal nanowires can be pre-coated with abarrier-forming corrosion inhibitor, e.g., BTA. In addition, the metalnanowires can also be treated with an anti-tarnish solution. Metalanti-tarnish treatments are known in the art. Specific treatmentstargeting H₂S corrosion are described in, e.g., U.S. Pat. No. 4,083,945,and U.S. Published Application No. 2005/0148480.

In yet other embodiments, the metal nanowires can be alloyed or platedwith another metal less prone to corrosion by atmospheric elements. Forexample, silver nanowires can be plated with gold, which is lesssusceptible to corroding by H₂S.

In certain embodiments, the transparent conductors described herein canbe fabricated by various coating methods, including sheet coating andhigh throughput web coating. In other embodiments, a laminating methodcan be used. Advantageously, the fabrication processes described hereindo not require vacuum deposition, in contrast to the current fabricationof the metal oxide films. Instead, the fabrication processes can becarried out using conventional solution-processing equipment. Moreover,the fabrication processes are compatible with directly patterning thetransparent conductor.

Nanowire Deposition and Transparent Conductor Fabrication

In certain embodiments, it is thus described herein a method offabricating a transparent conductor comprising: depositing a pluralityof metal nanowires on a substrate, the metal nanowires being dispersedin a fluid; and forming a metal nanowire network layer on the substrateby allowing the fluid to dry.

The metal nanowires can be prepared as described above. The metalnanowires are typically dispersed in a liquid to faciliate thedeposition. It is understood that, as used herein, “deposition” and“coating” are used interchangeably. Any non-corrosive liquid in whichthe metal nanowires can form a stable dispersion (also called “metalnanowires dispersion”) can be used. Preferably, the metal nanowires aredispersed in water, an alcohol, a ketone, ethers, hydrocarbons or anaromatic solvent (benzene, toluene, xylene, etc.). More preferably, theliquid is volatile, having a boiling point of no more than 200° C., nomore than 150° C., or no more than 100° C.

In addition, the metal nanowire dispersion may contain additives andbinders to control viscosity, corrosion, adhesion, and nanowiredispersion. Examples of suitable additives and binders include, but arenot limited to, carboxy methyl cellulose (CMC), 2-hydroxy ethylcellulose (HEC), hydroxy propyl methyl cellulose (HPMC), methylcellulose (MC), poly vinyl alcohol (PVA), tripropylene glycol (TPG), andxanthan gum (XG), and surfactants such as ethoxylates, alkoxylates,ethylene oxide and propylene oxide and their copolymers, sulfonates,sulfates, disulfonate salts, sulfosuccinates, phosphate esters, andfluorosurfactants (e.g., ZONYL® by DuPont).

In one example, a nanowire dispersion, or “ink” includes, by weight,from 0.0025% to 0.1% surfactant (e.g., a preferred range is from 0.0025%to 0.05% for Zonyl® FSO-100), from 0.02% to 4% viscosity modifier (e.g.,a preferred range is 0.02% to 0.5% for HPMC), from 94.5% to 99.0%solvent and from 0.05% to 1.4% metal nanowires. Representative examplesof suitable surfactants include Zonyl® FSN, Zonyl® FSO, Zonyl® FSH,Triton (x100, x114, x45), Dynol (604, 607), n-Dodecyl b-D-maltoside andNovek. Examples of suitable viscosity modifiers include hydroxypropylmethyl cellulose (HPMC), methyl cellulose, xanthan gum, polyvinylalcohol, carboxy methyl cellulose, hydroxy ethyl cellulose. Examples ofsuitable solvents include water and isopropanol.

If it is desired to change the concentration of the dispersion from thatdisclosed above, the percent of the solvent can be increased ordecreased. In preferred embodiments the relative ratios of the otheringredients, however, can remain the same. In particular, the ratio ofthe surfactant to the viscosity modifier is preferably in the range ofabout 80 to about 0.01; the ratio of the viscosity modifier to the metalnanowires is preferably in the range of about 5 to about 0.000625; andthe ratio of the metal nanowires to the surfactant is preferably in therange of about 560 to about 5. The ratios of components of thedispersion may be modified depending on the substrate and the method ofapplication used. The preferred viscosity range for the nanowiredispersion is between about 1 and 100 cP.

Optionally, the substrate can be pre-treated to prepare a surface tobetter receive the subsequent deposition of the nanowires. Surfacepre-treatments serve multiple functions. For example, they enable thedeposition of a uniform nanowire dispersion layer. In addition, they canimmobilize the nanowires on the substrate for subsequent processingsteps. Moreover, the pre-treatment can be carried out in conjunctionwith a patterning step to create patterned deposition of the nanowires.As will be discussed further in more detail below, pre-treatmentsinclude solvent or chemical washing, heating, deposition of anoptionally patterned intermediate layer to present an appropriatechemical or ionic state to the nanowire dispersion, as well as furthersurface treatments such as plasma treatment, UV-ozone treatment, orcorona discharge.

Following the deposition, the liquid is removed by evaporation. Theevaporation can be accelerated by heating (e.g., baking). The resultingnanowire network layer may require post-treatment to render itelectrically conductive. This post-treatment can be a process stepinvolving exposure to heat, plasma, corona discharge, UV-ozone, orpressure as described below.

In certain embodiments, it is thus described herein a method offabricating a transparent conductor comprising: depositing a pluralityof metal nanowires on a substrate, the metal nanowires being dispersedin a fluid; forming a metal nanowire network layer on the substrate byallowing the fluid to dry, coating a matrix material on the metalnanowire network layer, and curing the matrix material to form a matrix.

“Matrix material” refers to a material or a mixture of materials thatcan cure into the matrix, as defined herein. “Cure”, or “curing”, refersto a process where monomers or partial polymers (fewer than 150monomers) polymerize and/or cross-link so as to form a solid polymericmatrix. Suitable polymerization conditions are well known in the art andinclude by way of example, heating the monomer, irradiating the monomerwith visible or ultraviolet (UV) light, electron beams, and the like. Inaddition, “solidification” of a polymer/solvent system simultaneouslycaused by solvent removal is also within the meaning of “curing”.

In certain embodiments, the matrix material comprises a polymer.Optically clear polymers are known in the art. Examples of suitablepolymeric matrices include, but are not limited to: polyacrylics such aspolymethacrylates, polyacrylates and polyacrylonitriles, polyvinylalcohols, polyesters (e.g., polyethylene terephthalate (PET), polyesternaphthalate, and polycarbonates), polymers with a high degree ofaromaticity such as phenolics or cresol-formaldehyde (Novolacs®),polystyrenes, polyvinyltoluene, polyvinylxylene, polyimides, polyamides,polyamideimides, polyetherimides, polysulfides, polysulfones,polyphenylenes, and polyphenyl ethers, polyurethane (PU), epoxy,polyolefins (e.g. polypropylene, polymethylpentene, and cyclic olefins),acrylonitrile-butadiene-styrene copolymer (ABS), cellulosics, siliconesand other silicon-containing polymers (e.g. polysilsesquioxanes andpolysilanes), polyvinylchloride (PVC), polyacetates, polynorbornenes,synthetic rubbers (e.g. EPR, SBR, EPDM), and fluoropolymers (e.g.,polyvinylidene fluoride, polytetrafluoroethylene (TFE) orpolyhexafluoropropylene), copolymers of fluoro-olefin and hydrocarbonolefin (e.g., Lumiflon®), and amorphous fluorocarbon polymers orcopolymers (e.g., CYTOP® by Asahi Glass Co., or Teflon® AF by Du Pont).

In other embodiments, the matrix material comprises a prepolymer. A“prepolymer” refers to a mixture of monomers or a mixture of oligomersor partial polymers that can polymerize and/or crosslink to form thepolymeric matrix, as described herein. It is within the knowledge of oneskilled in the art to select, in view of a desirable polymeric matrix, asuitable monomer or partial polymer.

In a preferred embodiment, the prepolymer is photo-curable, i.e., theprepolymer polymerizes and/or cross-links upon exposure to irradiation.As will be described in more detail, matrices based on photo-curableprepolymers can be patterned by exposure to irradiation in selectiveregions. In other embodiments, the prepolymer is thermal-curable, whichcan be patterned by selective exposure to a heat source.

Typically, the matrix material is a liquid. The matrix material mayoptionally comprise a solvent. Any non-corrosive solvent that caneffectively solvate or disperse the matrix material can be used.Examples of suitable solvents include water, an alcohol, a ketone,tetrahydrofuran, hydrocarbons (e.g. cyclohexane) or an aromatic solvent(benzene, toluene, xylene, etc.). More preferrably, the solvent isvolatile, having a boiling point of no more than 200° C., no more than150° C., or no more than 100° C.

In certain embodiments, the matrix material may comprise a cross-linker,a polymerization initiator, stabilizers (including, for example,antioxidants, and UV stabilizers for longer product lifetime andpolymerization inhibitors for greater shelf-life), surfactants and thelike. In other embodiments, the matrix material may further comprise acorrosion inhibitor.

As noted herein, the transparent conductors can be fabricated by, forexample, sheet coating, web-coating, printing, and lamination.

(a) Sheet Coating

Sheet coating is suitable for coating a conductive layer on anysubstrate, in particular, rigid substrates.

FIGS. 13A-13B show an embodiment of the fabrication of the transparentconductor by sheet coating. A metal nanowires dispersion (not shown) canbe initially deposited to the substrate 14. A roller 100 can be rolledacross a top surface 105 of the substrate 14, leaving a metal nanowiresdispersion layer 110 on the top surface 105 (FIG. 13A). The layer 110 isallowed to dry and a metal nanowire network layer 114 is formed on thesurface 105 (FIG. 13B).

The substrate may require a pre-treatment to enable the deposition of auniform nanowire dispersion layer 110 that adheres to the substrate forsubsequent processing steps. This treatment can include solvent orchemical washing, heating, deposition of an optionally patternedintermediate layer to present an appropriate chemical or ionic state tothe nanowire dispersion, as well as further surface treatments such asplasma treatment, UV-ozone treatment, or corona discharge.

For example, an intermediate layer can be deposited on the surface ofthe substrate to immobilize the nanowires. The intermediate layerfunctionalizes and modifies the surface to facilitate the binding of thenanowires to the substrate. In certain embodiments, the intermediatelayer can be coated on the substrate prior to depositing the nanowires.In other embodiments, the intermediate layer can be co-deposited withthe nanowires.

In certain embodiments, multifunctional biomolecules such aspolypeptides can be used as the intermediate layer. Polypeptide refersto a polymeric sequence of amino acids (monomers) joined by peptide(amide) bonds. The amino acid monomers in a polypeptide can be the sameor different. Amino acids having side chain functionalities (e.g., aminoor carboxylic acid groups) are preferred. Examples of suitablepolypeptides thus include poly-L-lysine, poly-L-glutamic acid and thelike. The polypeptide can be coated on the substrate prior to thenanowire deposition. Alternatively, the polypeptide can be co-depositedon the substrate with the nanowire dispersion. Many substrates,including glass, polyester substrates (e.g., polyethylene terephthalate)exhibit affinities for polypeptides.

Advantageously, the intermediate layer can be deposited in apre-determined pattern, which enables the deposition of the nanowiresaccording to the same pattern.

Other pre-treatment methods can also be carried out in conjunction witha patterning step in order to perform patterned depositions. Forexample, plasma surface treatment can be carried out through an aperturemask having a desired pattern. The surface of the substrate thereforecomprises at least one pre-treated regions and at least one untreatedregion. Nanowires deposited on the pre-treated region adhere to thesubstrate better than they adhere to the untreated region. Accordingly,a patterned deposition can be achieved by removing the nanowires on theuntreated region by, e.g., washing.

It should be understood that the pre-treatments described above alsoapply to other methods of fabricating transparent conductors inaccordance with the description below.

The nanowire network layer formed may further require a post-treatmentto render it electrically conductive. This post-treatment can be aprocess step involving exposure to heat, plasma, corona discharge,UV-ozone, or pressure, as will be discussed in more detail below.

In some embodiments, a matrix material can be coated on the nanowirenetwork layer 114 to form a matrix material layer 116 (FIG. 13C). Asshown in FIG. 13D, the matrix material layer 116 is allowed to cure toobtain a matrix and the structures of FIGS. 10A-10E, can be obtained.

It is understood that a brush, a stamp, a spray applicator, a slot-dieapplicator or any other suitable applicator can be used in the place ofthe roller 100. Additionally, as discussed further below, reverse andforward gravure printing, slot die coating, reverse and forward beadcoating and draw down table can also be used to deposit nanowires onto asubstrate. Advantageously, a roller or stamp having recesses of apredetermined pattern can be used to coat a patterned metal nanowiresdispersion layer, or matrix material layer, thus printing a patternedconductive layer (e.g., Gravure printing). The conductive layer can alsobe patterned by spraying the nanowire or matrix formulation onto thesubstrate through an aperture mask. If the matrix material layer isdeposited or cured in a patterned layer, the pattern can be transferredinto the metal nanowire layer by removing sufficient numbers of them todrop the concentration of nanowires below the percolation threshold.Nanowires can be removed by washing or brushing them away with asuitable solvent or by transferring them to a tacky or adhesive roller.

It is further understood that additional depositions or coatings can becarried out, while allowing for drying or curing between two consecutivecoating step. For example, any number of the performance-enhancinglayers can be coated in the same manner as described above.

(b) Web Coating

Web-coating has been employed in the textile and paper industries forhigh-speed (high-throughput) coating applications. It is compatible withthe deposition (coating) processes for transparent conductorfabrication. Advantageously, web-coating uses conventional equipment andcan be fully automated, which dramatically reduces the cost offabricating transparent conductors. In particular, web-coating producesuniform and reproducible conductive layers on flexible substrates.Process steps can be run on a fully integrated line or serially asseparate operations.

FIG. 14A shows an embodiment in which a flexible substrate in the formof a film or web can be coated continuously along a moving path. Morespecifically, a substrate 14 mounted on reels 118 is drawn by a motor(not shown) and moves along a travelling path 120. The substrate can befed to the reels directly or via a conveyor belt system (not shown). Astorage tank 122 is positioned above the substrate 14. The storage tank122 contains a metal nanowires dispersion 124 for metal nanowiresdeposition. An aperture 128 in the storage tank 122 delivers acontinuous stream of metal nanowire dispersion 132 on the substrate 14to form a layer 110 on a top surface 105 of the substrate 14.

It is understood that the matrix material is stored in another storagetank (not shown), and the matrix material can be coated in the samemanner as described above.

It is further understood that any dispensing device can be used in theplace of the storage tank, including a spraying device (e.g., anatomizer that delivers pressurized dispersions), a brushing device, apouring device and the like. Like the sheet coating, a printing devicecan also be used to provide patterned coatings.

FIG. 14B shows an alternative method of web-coating in which the coatingis carried out on a bottom surface of a substrate. Like the methodillustrated in FIG. 14A, a substrate 14 moves along a traveling path120. A coating roller 140 is positioned below the substrate andpartially submerged in a metal nanowire dispersion 124 stored in astorage tank 122. The coating roller 140 delivers a metal nanowiredispersion layer 110 on a bottom surface 144 of the substrate 14.Coating roller 140 can rotate in the direction of the traveling path 120or in the opposite direction. The coating of the matrix material can becarried out in the same manner.

In the processes described in FIGS. 14A and 14B, it is noted thatvarious surface treatments can be applied prior to or after eachdeposition step. As will be described in more detail below, surfacetreatments can enhance the transparency and/or conductivity of theconductive layers formed. Suitable surface treatments include, but arenot limited to solvent or chemical washing, plasma treatments, Coronadischarge, UV/ozone treatment, pressure treatment and combinationsthereof.

FIG. 15A shows a comprehensive process flow for fabricating atransparent conductor. As shown, a web-coating system 146 includes atake-up reel 147 that is driven by a motor (not shown). The take up reel147 draws a substrate 14 (e.g., a flexible polymer film) from a supplyreel 148 along a traveling path 150. The substrate 14 is then subjectedto sequential treatments and coating processes along the traveling path150. It will become apparent to one skilled in the art that the speed ofthe reel, the speed of deposition, the concentration of the matrixmaterial, and the adequacy of the drying and curing processes are amongthe factors that determine the uniformity and the thickness of theconductive layer formed.

Moreover, in certain embodiments, pre-treatments are conducted toprepare the substrate for the subsequent coating processes. Morespecifically, the substrate 14 can be optionally surface-treated at apre-treatment station 160 to improve the efficiency of the subsequentnanowire deposition. In addition, surface treatment of the substrateprior to the deposition can enhance the uniformity of the nanowireslater deposited.

The surface treatment can be carried out by known methods in the art.For example, plasma surface treatment can be used to modify themolecular structure of the surface of the substrate. Using gases such asargon, oxygen or nitrogen, plasma surface treatment can create highlyreactive species at low temperatures. Typically, only a few atomiclayers on the surface are involved in the process, so the bulkproperties of the substrate (e.g. the polymer film) remain unaltered bythe chemistry. In many instances, plasma surface treatment providesadequate surface activation for enhanced wetting and adhesive bonding.As an illustrative example, oxygen plasma treatment can be carried outin a March PX250 system, using the following operating parameters: 150W, 30 sec, O₂ flow: 62.5 sccm, pressure: ˜400 mTorr.

In other embodiments, the surface treatment may include depositing anintermediate layer on the substrate. As noted above, the intermediatelayer typically exhibits affinities for both the nanowires and thesubstrate. Thus, the intermediate layer is capable of immobilizing thenanowires and causing the nanowires to adhere to the substrate.Representative materials suitable as the intermediate layer includemultifunctional biomolecules, including polypeptides (e.g.,poly-L-lysine.)

Other exemplary surface treatments include surface washing with asolvent, Corona discharge and UV/ozone treatment, all of which are knownto one skilled in the art.

The substrate 14 thereafter proceeds to a metal nanowires depositionstation 164, which delivers a metal nanowires dispersion 166, as definedherein. The deposition station can be a storage tank as described inFIG. 14A, a spraying device, a brushing device, and the like. A metalnanowires dispersion layer 168 is deposited on the surface 105.Alternatively, a printing device can be used to apply a patternedcoating of the metal nanowires dispersion on the substrate. For example,a stamp or roller having recesses of a predetermined pattern can beused. The stamp or roller can be continuously dipped into a metalnanowires dispersion by known methods in the art.

The layer 168 can be optionally rinsed at a rinsing station 172.Thereafter, the layer 168 is dried at a drying station 176 to form ametal nanowire network layer 180.

Optionally, the network layer 180 can be treated at a post-treatmentstation 184. For example, surface treatment of the metal nanowires withargon or oxygen plasma can improve the transparency and the conductivityof the network layer 180. As an illustrative example, Ar or N₂ plasmacan be carried out in a March PX250 system, using the followingoperating parameters: 300 W, 90 sec (or 45 sec), Ar or N₂ gas flow: 12sccm, pressure ˜300 mTorr. Other known surface treatments, e.g., Coronadischarge or UV/ozone treatment, may also be used. For example, theEnercon system can be used for a Corona treatment.

As a part of the post-treatment, the network layer can further bepressure treated. More specifically, the network layer 180 is fedthrough rollers 186 and 187, which apply pressure to the surface 185 ofthe network layer 180. It should be understood that a single rollercould also be used.

Advantageously, the application of pressure to a metal nanowire networkfabricated in accordance with a method described herein can increase theconductivity of the conducting layer.

In particular, pressure may be applied to one or both surfaces of aconducting sheet transparent conductor fabricated in accordance with amethod described herein by use of one or more rollers (e.g., cylindricalbars), one or both of which may, but need not, have a length dimensionlarger than a width dimension of the conducting layer. If a singleroller is used, the network layer may be placed on a rigid surface andthe single roller rolled across the exposed surface of the conductinglayer using known methods while pressure is applied to the roller. Iftwo rollers are used, the network layer may be rolled between the tworollers as shown in FIG. 15A.

In one embodiment, from 50 to 10,000 psi may be applied to thetransparent conductor by one or more rollers. It is also considered thatfrom 100 to 1000 psi, 200 to 800 psi or 300 to 500 psi may be applied.Preferably, pressure is applied to a transparent conductor prior to theapplication of any matrix material.

“Nip” or “pinch” rollers may be used if two or more rollers are used toapply pressure to the conducting sheet. Nip or pinch rollers are wellunderstood in the art and discussed in, for example, 3M TechnicalBulletin “Lamination Techniques for Converters of Laminating Adhesives,”March, 2004, which is hereby incorporated by reference in its entirety.

It was determined that application of pressure to a metal nanowirenetwork layer improved the conductivity thereof either before or afterapplication of a plasma treatment as discussed above, and may be donewith or without a previous or subsequent plasma treatment. As shown inFIG. 15A, the rollers 186 an 187 may be rolled across the surface 185 ofthe network layer 180 a single or multiple times. If the rollers arerolled across the network layer 180 multiple times, the rolling may becarried out in the same direction with respect to an axis parallel tothe rolled surface of the sheet (e.g., along the traveling path 150) orin different directions (not shown).

FIG. 15B is an SEM image of a portion of a metal nanowire conductivenetwork 810 after application of from about 1000 psi to about 2000 psiusing a stainless steel roller. Conductive network 810 includes aplurality of nanowire crossing points such as crossing points 812 a, 812b and 812 c. As shown, at least the top nanowires 814, 816, and 818 ateach of crossing points 812 a, 812 b and 812 c have flattened crosssections where the intersecting wires have been pressed into each otherby the application of pressure, thereby enhancing the connectivity aswell as the conductivity of the nanowire conductive network.

The application of heat may also be used at this point as apost-treatment. Typically, the transparent conductor exposed to anywherefrom 80° C. to 250° C. for up to 10 min, and more preferably is exposedto anywhere from 100° C. to 160° C. for anywhere from about 10 secondsto 2 minutes. The transparent conductor can also be exposed totemperatures higher than 250° C. and can be as high as 400° C.,depending on the type of substrate. For example, glass substrate can beheat-treated at a temperature range of about 350° C. to 400° C. However,post treatments at higher temperatures (e.g., higher than 250° C.) mayrequire the presence of a non-oxidative atmosphere, such as nitrogen ora noble gas.

The heating can be carried out either on-line or off-line. For example,in an off-line treatment, the transparent conductor can be placed in asheet drying oven set at a given temperature for a predetermined amountof time. Heating a transparent conductor in such a way can improve theconductivity of a transparent conductor fabricated as described herein.For example, a transparent conductor fabricated using a reel-to-reelprocess as described herein was placed in a sheet drying oven set at atemperature of 200° C. for 30 seconds. Before this heat post-treatment,the transparent conductor had a surface resistivity of about 12 kΩ/□,which dropped to about 58Ω/□ after the post-treatment.

In another example, a second, similarly prepared transparent conductorwas heated in a sheet oven at 100° C. for 30 seconds. The resistivity ofthe second transparent conductor dropped from about 19 kΩ/□ to about400Ω/□. It is also considered that the transparent conductor may beheated using methods other than a sheet oven. For example, an infraredlamp could be used as either an in-line or off-line method to heat thetransparent conductor. RF currents may also be used to heat the metalnanowire network. RF currents may be induced in a metal nanowire networkby either broadcast microwaves or currents induced through electricalcontacts to the nanowire network.

Additionally, a post-treatment that applies both heat and pressure tothe transparent conductor can be used. In particular, to apply pressure,the transparent conductor can be placed through one or more rollers asdescribed above. To simultaneously apply heat, the rollers may beheated. The pressure applied by the rollers is preferably from 10 to 500psi and more preferably from 40 to 200 psi. Preferably, the rollers areheated to between about 70° C. and 200° C. and more preferably tobetween about 100° C. and 175° C. Such application of heat incombination with pressure can improve the conductivity of a transparentconductor. A machine which may be used to apply both appropriatepressure and heat simultaneously is a laminator by Banner AmericanProducts of Temecula, Calif. Application of heat in combination withpressure can be done either before or after deposition and curing of amatrix or other layers as described below.

Another post-treatment technique that can be used to increaseconductivity of a transparent conductor is to expose the metal wireconductive network of a transparent conductor fabricated as disclosedherein to a metal reducing agent. In particular, a silver nanowireconductive network can preferably be exposed to a silver reducing agentsuch as sodium borohydride for, preferably, anywhere from about 10seconds to about 30 minutes, and more preferably from about 1 minute toabout 10 minutes. As would be understood by one of ordinary skill in theart, such exposure can be done either in-line or off-line.

As noted above, such a treatment can increase the conductivity of atransparent conductor. For example, a transparent conductor of silvernanowires on a substrate of PET and prepared according to a reel-to-reelmethod disclosed herein was exposed to 2% NaBH₄ for 1 minute, which wasthen rinsed in water and dried in air. Before this post-treatment thetransparent conductor had a resistivity of about 134Ω/□ and after thispost-treatment, the transparent conductor had a resistivity of about9Ω/□. In another example, a transparent conductor of silver nanowires ona glass substrate was exposed to 2% NaBH₄ for 7 minutes, rinsed in waterand air dried. Before this post-treatment the transparent conductor hada resistivity of about 3.3 MΩ/□0 and after this post-treatment, thetransparent conductor had a resistivity of about 150Ω/□. Reducing agentsother than sodium borohydride can be used for this post treatment. Othersuitable reducing agents include other borohydrides such as sodiumborohydride; boron nitrogen compounds such as dimethyl aminoborane(DMAB); and gas reducing agents, such as hydrogen gas (H₂).

Thereafter, the substrate 14 proceeds to a matrix deposition station188, which delivers a matrix material 190, as defined herein. The matrixdeposition station 188 can be a storage tank as described in FIG. 14A, aspraying device, a brushing device, a printing device and the like. Alayer of the matrix material 192 is thus deposited on the network layer180. Advantageously, the matrix material can be deposited by a printingdevice to form a patterned layer.

The layer 192 is then allowed to cure at a curing station 200. Where thematrix material is a polymer/solvent system, the layer 192 can be curedby allowing the solvent to evaporate. The curing process can beaccelerated by heating (e.g., baking). When the matrix materialcomprises a radiation-curable prepolymer, the layer 192 can be cured byirradiation. Depending on the type of the prepolymer, thermal curing(thermally induced polymerization) can also be used.

Optionally, a patterning step can be carried out before the layer of thematrix material 192 is cured. A patterning station 198 can be positionedafter the matrix deposition station 188 and before the curing station200. The patterning step will be discussed in more detail below.

The curing process forms a conductive layer 204 comprising the metalnanowires network layer 180 in a matrix 210. The conductive layer 204can be further treated at a post-treatment station 214.

In one embodiment, the conductive layer 204 can be surface treated atthe post-treatment station 214 to expose a portion of the metalnanowires on the surface of the conductive layer. For example, a minuteamount of the matrix can be etched away by solvent, plasma treatment,Corona discharge or UV/ozone treatments. Exposed metal nanowires areparticularly useful for touch screen applications.

In another embodiment, a portion of the metal nanowires is exposed onthe surface of the conductive layer 204 following the curing process(see, also, FIGS. 10C and 10D), and an etching step is not needed. Inparticular, when the thickness of the matrix material layer 192 andsurface tension of the matrix formulation are controlled appropriately,the matrix will not wet the top portion of the metal nanowire networkand a portion of the metal nanowires will be exposed on the surface ofthe conductive layer.

The conductive layer 204 and the substrate 14 are then drawn up by thetake-up reel 147. This flow process of fabrication is also referred toas a “reel-to-reel” or “roll-to-roll” process. Optionally, the substratecan be stabilized by traveling along a conveyor belt.

In the “reel-to-reel” process, multiple coating steps can be carried outalong the traveling path of a moving substrate. The web coating system146 therefore can be customized or otherwise adapted to incorporate anynumber of additional coating stations as needed. For example, coatingsof the performance-enhancing layers (anti-reflective, adhesive, barrier,anti-glare, protective layers or films) can be fully integrated into theflow process.

Advantageously, the reel-to-reel process is capable of producing uniformtransparent conductors at high-speed and low cost. In particular, due tothe continuous flow of the coating process, the coated layers do nothave trailing edges.

(c) Lamination

Despite its versatility, the “reel-to-reel” process is not compatiblewith a rigid substrate, such as glass. While rigid substrates can becoated by sheet coating and can possibly be carried on a conveyor belt,they typically experience edge defects and/or lack of uniformity. Inaddition, sheet coating is a lower throughput process, which cansignificantly increase the cost of production.

Thus, it is described herein a lamination process for fabricating atransparent conductor through the use of a flexible donor substrate.This process is compatible with both rigid substrates and flexiblesubstrates. More specifically, the lamination process comprises: coatinga conductive layer on a flexible donor substrate, the conductive layerincluding a plurality of metal nanowires which can be embedded in amatrix; separating the conductive layer from the flexible donorsubstrate; and transferring the conductive layer to a substrate ofchoice. Advantageously, the coating steps onto the flexible donorsubstrate can be carried out by a reel-to-reel process because the donorsubstrate is flexible. The conductive layer formed thereof can then betransferred to a substrate of choice, which can be rigid or flexible,through standard lamination processes. If only nanowires are depositedonto the flexible donor substrate and no matrix material is used, alamination adhesive may be used to attach the conductive layer to thesubstrate of choice.

“Flexible donor substrate” refers to a flexible substrate in the form ofa sheet, film, web, and the like. The flexible donor substrate is notparticularly limited so long as it can be separated from the conductivelayer. The flexible donor substrate can be any of the flexiblesubstrates as described herein. In addition, the flexible donorsubstrate can be woven or non-woven textile, paper, and the like. Theflexible donor substrate need not be optically clear.

In certain embodiments, the flexible donor substrate can be pre-coatedwith a release layer prior to the coating of the conductive layer.“Release layer” refers to a thin layer adhered to the donor substrateand onto which a conductive layer can be formed by web coating. Therelease layer must allow for an easy removal of the donor substrate fromthe conductive layer without damaging the conductive layer. Typically,the release layer is formed of a material having low surface energy,including but not limited to: silicon based polymers, fluorinatedpolymers, starch, and the like.

FIG. 16A illustrates an example of a laminated structure 230 comprising,a flexible donor substrate 240, a release layer 244 coated on theflexible donor substrate 240, and a conductive layer 250 coated on therelease layer 244.

The laminated structure 230 can be fabricated in the same manner asdescribed in connection with FIG. 15A, using the flexible donorsubstrate. Prior to the metal nanowire deposition, the release layer 244is deposited or coated on the flexible donor substrate. The conductivelayer 250 can be formed by metal nanowires deposition followed by matrixdeposition, as described herein.

The conductive layer is then uniformly transferred to a substrate ofchoice. In particular, a rigid substrate (e.g., glass), which istypically not adaptable to the reel-to-reel coating process, can belaminated with the conductive layer. As shown in FIG. 16B, the laminatedstructure 230 is transferred to a substrate 260 (e.g., glass) bycontacting a surface 262 of the conductive layer 250 to the substrate260. In certain embodiments, the polymeric matrix (e.g., PET, PU,polyacrylates) provides adequate adhesion to the substrate 260.Thereafter, as shown in FIG. 16C, the flexible donor substrate 240 canbe removed by detaching the release layer 244 from the conductive layer250.

In other embodiments, an adhesive layer can be used to provide a betterbonding between the conductive layer and the substrate during thelamination step. FIG. 17A shows a laminated structure 270 comprising, inaddition to the flexible donor substrate 240, the release layer 244 andthe conductive layer 250, an overcoat 274 and an adhesive layer 278. Theadhesive layer 278 has an adhesive surface 280.

The laminated structure 270 can be fabricated by a reel-to-reel processas described in connection with FIG. 15A, with the understanding thatthe web coating system 146 is adapted to provide additional stations forcoating the adhesive layer and the overcoat. The adhesive layer is asdefined herein (e.g., polyacrylates, polysiloxanes), and can be pressuresensitive, hot-melted, radiation-curable, and/or thermally curable. Theovercoat can be one or more of the performance-enhancing layers,including a hard coat, an anti-reflective layer, an protective film, abarrier layer, and the like.

In FIG. 17B, the laminated structure 270 is bonded with the substrate260 via the adhesive surface 280. Thereafter, as shown in FIG. 17C, theflexible donor substrate 240 is removed by detaching the release layer244 from the overcoat 274.

In certain embodiments, heat or pressure can be used during thelamination process to enhance the bonding between the adhesive layer (orthe conductive layer in the absence of an adhesive layer) and thesubstrate.

In other embodiments, a release layer is not necessary due to anaffinity differential of the conductive layer with respect to theflexible donor substrate and the substrate of choice. For example, theconductive layer may have a much higher affinity to glass than to atextile donor substrate. After the lamination process, the textile donorsubstrate can be removed while the conductive layer is firmly bondedwith the glass substrate.

In certain embodiments, a patterned transfer is possible during thelamination process. For example, the substrate can be heated by athermal gradient, which affords heated regions and unheated regions onthe substrate according to a predetermined pattern. Only the heatedregions will be laminated with the conductive layer due to an enhancedaffinity (e.g., adhesion), therefore providing a patterned conductivelayer on the substrate. Heated regions on a substrate can be generated,for example, by a nichrome wire heater positioned beneath the areas of asubstrate to be heated.

In other embodiments, a patterned transfer can be affected by a pressuregradient based on a pressure-sensitive affinity displayed by certainmatrix materials or adhesives. For example, a patterned laminatingroller can be used to apply different pressures according to apredetermined pattern. The patterned laminating roller can also beheated to further the affinity differential between pressured region andunpressured region.

In yet other embodiments, the conductive layer can be pre-cut (e.g., diecut) according to a predetermined pattern, prior to the laminationprocess. After transferring the pre-cut conductive layer to thesubstrate, the conductive layer of the predetermined pattern is retainedwhile the rest is removed along a pre-cut contour.

Patterning

As noted above, a patterned conductive layer can be formed byselectively curing a prepolymer coating according to a pattern. Thecuring process can be carried out photolytically or thermally. FIG. 18illustrates an embodiment in which a conductive layer isphoto-patterned. More specifically, the metal nanowire network layer 114is deposited on the substrate 14 according to a method described herein(e.g., FIGS. 13A-13D). It should be understood that the substrate 14 canbe any substrate, including a flexible donor substrate.

Thereafter, a prepolymer coating 300 is deposited on the network layerof metal nanowires 114. An irradiation source 310 provides the photonenergy for curing the prepolymer coating. A mask 314 is positionedbetween the prepolymer coating 300 and the irradiation source 310. Uponexposure, only regions exposed to the irradiation are cured (i.e.,regions 320); the prepolymer coating and nanowires in the uncuredregions 324 can be removed by washing or brushing with a suitablesolvent or by lifting them off with a tacky roller.

Photo-curable prepolymers are well known in the art. In certainembodiments, the photo-curable prepolymer includes a monomer comprisingone or more double bonds or functional groups, e.g. hydrides or hydroxylgroups, suitable for chain extension and crosslinking. In otherembodiments, the photo-curable prepolymer comprises a partial polymer oroligomer that contains one or more double bonds or functional groups,e.g. hydrides or hydroxyls, suitable for cross-linking or chainextension.

Examples of monomers containing a double bond are alkyl or hydroxyalkylacrylates or methacrylates, such as methyl, ethyl, butyl, 2-ethylhexyland 2-hydroxyethyl acrylate, isobornyl acrylate, methyl methacrylate andethyl methacrylate, silicone acrylates, acrylonitrile, acrylamide,methacrylamide, N-substituted (meth)acrylamides, vinyl esters such asvinyl acetate, vinyl ethers such as isobutyl vinyl ether, styrene,alkyl- and halostyrenes, N-vinylpyrrolidone, vinyl chloride andvinylidene chloride.

Examples of monomers containing two or more double bonds are thediacrylates of ethylene glycol, propylene glycol, neopentyl glycol,hexamethylene glycol and of bisphenol A, and4,4′-bis(2-acryloyloxyethoxy)diphenylpropane, trimethylolpropanetriacrylate, pentaerythritol triacrylate or tetraacrylate, vinylacrylate, divinylbenzene, divinyl succinate, diallyl phthalate, triallylphosphate, triallyl isocyanurate or tris(2-acryloylethyl) isocyanurate.

Examples of partial polymers include, but are not limited to,acrylicized epoxy resins, acrylicized polyesters, polyesters containingvinyl ether or epoxy groups, polyurethanes and polyethers, unsaturatedpolyester resins. In a preferred embodiment, the prepolymer is anacrylic.

Optionally, a photo-initiator can be used to initiate the polymerizationand/or crosslinking reactions. The photo-initiator absorbs the photonenergy and produces radicals, which initiates a cascade of radicalpolymerization, including chain-extension and cross-linking.Photo-initiators are well known in the art. Examples of suitablephoto-initiators include, but are not limited to, oxime esters, phenylketones, onium salts, and phosphine oxides, see, e.g. U.S. Pat. Nos.6,949,678, 6,929,896, and 6,803,392; N. Buhler & D. Bellus,“Photopolymers as a powerful tool in modern technology”, Pure & Appl.Chem., Vol. 67, No. 1, pp. 25-31, 1995; J. Crivello in Advances inPolymer Science, Vol. 62, pp. 1-48 (1984). In a preferred embodiment,the photo-initiator is Ciba Irgacure™ 754. Typically, with the use ofthe photo-initiator, the prepolymer coating can cure within 5 minutes,more preferably within 30 seconds.

In other embodiments, thermal-patterning can be carried out using aninsulating thermal mask (e.g., an aperture mask), which only exposesregions of a matrix material layer to be cured to a heat source.Alternatively, in a mask-less approach, laser direct-write technologycan be used to directly “write” a heated pattern on the prepolymercoating layer. Thermally-curable matrix materials are known to oneskilled in the art. For example, the matrix material can be an epoxy, aresin and a sol-gel composite material.

Both the photo-patterning method and thermal-patterning method arecompatible with the “reel-to-reel” process described above. For example,a photo-patterning station 198 can be a part of the web coating system146, as shown in FIG. 15A. The photo-patterning station 198 can beconfigured in a number of ways to allow for continuous exposure andcuring of the prepolymer coating.

In one embodiment, as shown in FIG. 19A, a rotating cylinder 330 is partof the photo-patterning station 198 (the web coating system 146 is notshown). The substrate 14, coated with a prepolymer coating 300, is movedalong by a conveyor belt 332. The rotating cyclinder rotates at the samespeed as the conveyor belt 332. The irradiation source 310 is positionedwithin the rotating cyclinder 330. An exterior 334 of the rotatingcyclinder 330 is patterned, perforated or otherwise provided withopenings 338 to allow the light to irradiate the prepolymer coating 300.Optionally, a guard slit or a collimator 340 for preventing any straylight can be positioned closely above the moving substrate.

In a related configuration, as shown in FIG. 19B, a patterning belt 350having a patterned or perforated exterior 352 can be used. Thepatterning belt 350 is driven by rollers 354, one of which is connectedto a motor (not shown). The patterning belt 350 moves at the same speedas the moving conveyor belt 332, allowing continuous exposure of theprepolymer coating 300 to the irradiation source 310 through openings360. Optionally, a guard slit 340 can be used.

FIG. 20 shows a partially integrated system 400 for forming a patternedconductive layer on a substrate. The system 400 can be fully integratedinto the web coating system 146. In particular, the photo-patterningstation 198 is identical to the one shown in FIG. 19A. Following thephoto exposure and curing, the prepolymer coating 300 is cured atselective regions and will be further treated at a washing station 370to remove any uncured prepolymer. The substrate 14, now comprising curedregions 380 and bare metal nanowires regions 374, moves to a rotatingtacky roller 384. The tacky roller 384 contacts and removes the baremetal nanowires regions 374. Following the removal of the bare metalnanowires, the substrate is coated with conductive regions 380 amongnon-conductive regions 386.

The transparent conductors as described herein can be used as electrodesin a wide variety of devices, including any device that currently makesuse of transparent conductors such as metal oxide films. Examples ofsuitable devices include flat panel displays, LCDs, touch screens,electomagnetic shieldings, functional glasses (e.g., for electrochromicwindows), optoelectronic devices, and the like. In addition, thetransparent conductors herein can be used in flexible devices, such asflexible displays and touch screens.

In one embodiment, the transparent conductors can be used as the pixelelectrodes in a liquid crystal display (LCD) device. FIG. 21 showsschematically an LCD device 500. A backlight 504 projects light througha polarizer 508 and a bottom glass substrate 512. A plurality of firsttransparent conductor strips 520 are positioned between the bottom glasssubstrate 512 and a first alignment layer 522. Each transparentconductor strip 520 alternates with a data line 524. Spacers 530 areprovided between the first alignment layer 522 and a second alignmentlayer 532, the alignment layers sandwiching liquid crystals 536 inbetween. A plurality of second transparent conductor strips 540 arepositioned on the second alignment layer 532, the second transparentconductor strips 540 orienting at a right angle from the firsttransparent conductor strips 520. The second transparent conductorstrips 540 are further coated with a passivation layer 544, coloredmatrices 548, a top glass substrate 550 and a polarizer 554.Advantageously, the transparent conductor strips 520 and 540 can bepatterned and transferred in a laminating process onto the bottom glasssubstrate, and the alignment layer, respectively. Unlike theconventionally employed metal oxide strips (ITO), no costly depositionor etching processes are required.

In a further embodiment, the transparent conductor described hereinforms part of a touch screen. A touch screen is an interactive inputdevice integrated onto an electronic display, which allows a user toinput instructions by touching the screen. The touch screen is opticallyclear to allow light and image to transmit through.

FIG. 22 shows schematically a touch screen device 560 according to oneembodiment. The device 560 includes a first transparent conductor 564comprising a first substrate 568 coated or laminated with a firstconductive layer 572, the first conductive layer 572 having a topconductive surface 576. A second transparent conductor 580 is positionedabove the first transparent conductor 564 and separated therefrom byadhesive enclosures 584 and 584′ at respective ends of the device 560.The second transparent conductor 580 includes a second conductive layer588 coated or laminated on a second substrate 592. The second conductivelayer 588 has an inner conductive surface 594 facing the top conductivesurface 576 and is suspended over spacers 596.

When a user touches the second transparent conductor 580, the innerconductive surface 594 contacts the top conductive surface 576 of thefirst transparent conductor 564 and causes a change in the electrostaticfield. A controller (not shown) senses the change and resolves theactual touch coordinate, which information is then passed to anoperating system.

According to this embodiment, the inner conductive surface 594 and thetop conductive surface 576 each has surface resistivity in the range ofabout 10-1000Ω/□, more preferably, about 10-500Ω/□. Optically, the firstand second transparent conductors have high transmission (e.g., >85%) toallow for images to transmit through.

The first and second substrates can be a variety of materials, asdescribed herein. For example, the first substrate can be rigid (e.g.,glass, or rigid plastic such as polycarbonate or polyacrylate) while thesecond substrate can be a flexible film. Alternatively, for a flexibletouch screen application, both substrates can be flexible films (e.g.,plastic).

Currently available touch screens typically employ metal oxideconductive layers (e.g., ITO films). As note above, ITO films arefragile and costly to fabricate. In particular, ITO films are typicallydeposited on glass substrates at high temperature and in vacuo. Incontrast, the transparent conductors described herein can be fabricatedby high throughput methods and at low temperatures. They also allow formore diverse substrates, including flexible and durable substrates suchas plastic films.

The transparent conductor structures, their electrical and opticalproperties, and the methods of fabrication are illustrated in moredetail by the following non-limiting examples.

EXAMPLES Example 1 Synthesis of Silver Nanowires

Silver nanowires were synthesized by the reduction of silver nitratedissolved in ethylene glycol in the presence of poly(vinyl pyrrolidone)(PVP) following the “polyol” method described in, e.g. Y. Sun, B. Gates,B. Mayers, & Y. Xia, “Crystalline silver nanowires by soft solutionprocessing”, Nanoletters, (2002), 2(2) 165-168. A modified polyolmethod, described in U.S. Provisional Application No. 60/815,627 in thename of Cambrios Technologies Corporation, produces more uniform silvernanowires at higher yields than does the conventional “polyol” method.This application is incorporated by reference herein in its entirety.

Example 2 Preparation of a Transparent Conductor

An Autoflex EBG5 polyethylene terephthalate (PET) film 5 μm thick wasused as a substrate. The PET substrate is an optically clear insulator.The light transmission and haze of the PET substrate are shown inTable 1. Unless specified otherwise, the light transmission was measuredusing the methodology in ASTM D1003.

An aqueous dispersion of silver nanowires was first prepared. The silvernanowires were about 70 nm to 80 nm in width and around 8 μm in length.The concentration of the silver nanowires (AgNW) was about 0.5% w/v ofthe dispersion, resulting in an optical density of about 0.5 (measuredon a Molecular Devices Spectra Max M2 plate reader). The dispersion wasthen coated on the PET substrate by allowing the nanowires to sedimentonto the substrate. As understood by one skilled in the art, othercoating techniques can be employed e.g., flow metered by a narrowchannel, die flow, flow on an incline and the like. It is furtherunderstood that the viscosity and shear behavior of the fluid as well asthe interactions between the nanowires may affect the distribution andinterconnectivity of the nanowires coated.

Thereafter, the coated layer of silver nanowires was allowed to dry bywater evaporation. A bare silver nanowire film, also referred to as a“network layer”, was formed on the PET substrate. (AgNW/PET) The lighttransmission and haze were measured using a BYK Gardner Haze-gard Plus.The surface resistivity was measured using a Fluke 175 True RMSMultimeter. The results are shown in Table 1. The interconnectivity ofthe nanowires and an areal coverage of the substrate can also beobserved under an optical or scanning electron microscope.

The matrix material was prepared by mixing polyurethane (PU) (MinwaxFast-Drying Polyurethane) in methyl ethyl ketone (MEK) to form a 1:4(v/v) viscous solution. The matrix material was coated on the baresilver nanowire film spin-coating. Other known methods in the art, forexample, doctor blade, Meyer rod, draw-down or curtain coating, can beused. The matrix material was cured for about 3 hours at roomtemperature, during which the solvent MEK evaporated and the matrixmaterial hardened. Alternatively, the curing can take place in an oven,e.g., at a temperature of 50° C. for about 2 hours.

A transparent conductor having a conductive layer on the PET substrate(AgNW/PU/PET) was thus formed. The conductive layer of the silvernanowires in the matrix was about 100 nm thick. Its optical andelectrical properties were measured and the results are shown in Table1.

The transparent conductor was further subjected to a tape test. Morespecifically, a 3M Scotch® 600 adhesive tape was firmly applied to thesurface of the matrix and then removed. Any loose silver nanowires wereremoved along with the tape. After the tape test, the optical andelectrical properties of the transparent conductor were measured and theresults are shown in Table 1.

By way of comparison, a matrix-only film was formed on a PET substrate(PU/PET) under the same conditions as described above. The opticalproperties (light transmission and haze) and the electrical propertiesof the PU/PET are also provided in Table 1.

As shown in Table 1, the matrix-only film on PET (PU/PET) had a slightlyhigher light transmission as well as haze value than a PET substrate.Neither was conductive. By comparison, the bare silver nanowire film onPET was highly conductive, registering a surface resistivity of 60Ω/□.The deposition of the bare silver nanowire film on the PET lowered thelight transmission and increased the haze. However, the bare silvernanowire film on PET was still considered optically clear with a lighttransmission of more than 80%. The optical and electrical properties ofthe bare silver nanowire film on PET were comparable or superior tometal oxide films (e.g., ITO) formed on PET substrates, which typicallyrange from 60 to 400 Ω/□.

As further shown in Table 1, the transparent conductor based on silvernanowires in the polyurethane matrix had an almost identical lighttransmission as the bare silver nanowire film on PET, and a slightlyhigher haze. The resistivity of the transparent conductor remained thesame as the bare silver nanowire film, indicating that the coating ofthe matrix material did not disturb the silver nanowire film. Thetransparent conductor thus formed was optically clear, and had acomparable or superior surface resistivity to metal oxide films (e.g.,ITO) formed on PET substrates.

In addition, the tape test did not alter the resistivity or the lighttransmission of the transparent conductor, and only slightly increasedthe haze.

TABLE 1 Transparent Resistivity Media Light Transmission (%) Haze (%)(Ω/□) PET 91.6 0.78 non-conductive PU/PET 92.3 0.88 non-conductiveAgNW/PET 87.4 4.76 60 AgNW/PU/PET 87.2 5.74 60 After tape test 87.2 5.9460

Example 3 Accelerated H₂S Corrosion Tests

Sulfides, such as hydrogen sulfide (H₂S), are known corrosive agents.The electrical properties of the metal nanowires (e.g., silver) canpotentially be affected in the presence of the atmospheric sulfides.Advantageously, the matrix of the transparent conductor serves as a gaspermeation barrier. This prevents, to certain degree, the atmosphericH₂S from contacting the metal nanowires embedded in the matrix.Long-term stability of the metal nanowires can be further obtained byincorporating one or more corrosion inhibitors in the matrix, asdescribed herein.

In the United States, the amount of H₂S in the air is about 0.11-0.33parts per billion (ppb). At this level, corrosion is expected to takeplace over an extended period of time. Thus, an accelerated H₂Scorrosion test was designed to provide an extreme case of H₂S corrosion.

Freshly cooked egg yolks were broken into pieces and sealed in a plasticbag. A H₂S meter (Industrial Scientific, GasBadge Plus—Hydrogen SulfideSingle Gas Monitor) was inserted into the bag to monitor the release ofH₂S from the egg yolks. FIG. 23 shows a typical release profile of theH₂S gas over a period of 24 hours. After an initial build-up of the H₂Sin the bag, the gas level dropped, indicating that the gas had diffusedout of the permeable bag. Nevertheless, the levels of the H₂S gas(peaked at 7.6 ppm) in the bag greatly surpassed the level of theatmospheric H₂S gas.

A bare silver nanowire film on PET was prepared according to Example 2.The film was placed in a plastic bag with freshly cooked egg yolks. Thefilm began to darken within two hours, indicating that the silver hadbeen tarnished and black Ag₂S was formed. In contrast, color changes infilms of silver nanowires in polyurethane matrix were not observed untilafter 2-3 days, indicating that the polyurethane matrix acted as abarrier to slow down the permeation of the H₂S gas.

Example 4 Incorporation of Corrosion Inhibitors

The following samples of conductive films were prepared. A PET substratewas used for each sample. In certain samples, corrosion inhibitors,including benzotriazole, dithiothiadiazole and acrolein, wereincorporated during the preparation of the conductive films.

Samples 1-2 were prepared according to the method described herein. Nocorrosion inhibitor was present.

Sample 1 was a conductive film of bare silver nanowires.

Sample 2 was a conductive film of silver nanowires in a polyurethanematrix.

Samples 3-6 were prepared by first forming a bare silver nanowire filmon a PET substrate (i.e., Sample 1). Thereafter, various corrosioninhibitors were incorporated during the coating processes of the matrixmaterial.

Sample 3 was prepared by coating a 0.1 w/v % solution of benzotriazole(BTA) in methyl ethyl ketone (MEK) on the bare silver nanowire film,allowing the solvent to dry after coating, followed by coating thematrix material of polyurethane (PU) in MEK (1:4).

Sample 4 was prepared by first incorporating 1.5 v/v % ofdithiothiadiazole in a matrix material PU/MEK (1:4), followed by coatingthe matrix material on the bare silver nanowire film.

Sample 5 was prepared by first dipping a bare silver nanowire film in asolution of 1.5 v/v % of dithiothiadiazole in MEK, allowing the solventto dry after dipping, followed by coating with a matrix material PU/MEK(1:4) having 1.5 v/v % of dithiothiadiazole.

Sample 6 was prepared by first incorporating 1.5 v/v % of acrolein in amatrix material PU/MEK (1:4), followed by coating the matrix material ona bare silver nanowire film.

The optical and electrical properties of the Samples 1-6 were measuredbefore and after an accelerated H₂S treatment, as described in Example3. The results are shown in FIGS. 24A, 24B and 24C.

FIG. 24A shows the light transmission measurements of Samples 1-6 beforethe H₂S treatment and 24 hours after the H₂S treatment. For purpose ofcomparison, the decrease of light transmission for each sample is alsographed. Prior to the H₂S treatment, all of the samples were shown to beoptically clear (having a light transmission higher than 80%). Following24 hours of the H₂S treatment, all of the samples have experienceddecreases in their light transmissions due to different degrees ofsilver tarnish.

As expected, Sample 1 had the most reduction in light transmission.Samples 3 and 6 did not perform better than the matrix-only sample(Sample 2). Samples 4 and 5, however, had less reduction in lighttransmission compared to the matrix-only sample, indicating thecorrosion inhibitor dithiothiadiazole was effective in protecting thesilver nanowires from being corroded.

FIG. 24B shows the resistance measurements of Samples 1-6 before the H₂Streatment and 24 hours after the H₂S treatment. For purpose ofcomparison, the decrease of the resistance for each sample is alsographed. As shown, all but Sample 4 experienced dramatic increases intheir resistances and effectively became non-conductive, although theonset of the degradation in electrical properties was significantlydelayed for some samples. Sample 4 had only a modest increase in itsresistance. It is noted that the impacts of H₂S on Sample 4 and Sample 5differed considerably, despite that both Samples had the same corrosioninhibitor (dithiothiadiazole). This implies that the coating processesmay affect the effectiveness of a given corrosion inhibitor.

FIG. 24C shows the haze measurements of Samples 1-6 before the H₂Streatment and 24 hours after the H₂S treatment. For purpose ofcomparison, the change in the haze for each sample is also graphed. Allthe samples showed increases in their haze measurements. With theexception of Samples 1 and 6, the haze was within acceptable range (lessthan 10%) for each of Samples 2-5.

Sample 4 was shown to have the best overall performance in withstandingthe corrosive H₂S gas. By incorporating the corrosion inhibitor(dithiothiadiazole) in the matrix, the transparent conductor showedclear advantage over Sample 2 in which no corrosion inhibitor waspresent.

It is noted that the H₂S levels in these accelerated tests were fargreater than the atmospheric H₂S. It is therefore expected thattransparent conductors prepared similarly as Sample 4 would fare evenbetter in the presence of the atmospheric H₂S.

Example 5 Pressure-Treatment of Metal Nanowire Network Layers

Table 2 illustrates the results of two trials of applying pressure to asurface of a silver nanowire network layer (or “network layer”) on asubstrate.

Specifically, silver nanowires of around 70 nm to 80 nm in width andaround 8 μm in length were deposited on an Autoflex EBG5 PET substrate.The substrate was treated with Argon plasma prior to the deposition ofthe nanowires. A network layer was formed according to the methoddescribed in Example 2. No matrix material was applied to the networksprior to the pressure treatment. The Trials listed in Table 2 werecarried out using a single stainless steel roller on a rigid bench-topsurface. The area of the network layer treated was from 3 to 4 incheswide and from 3 to 4 inches long.

TABLE 2 Trial 1 Trial 2 Transmission Process R (Ω/square) R (Ω/square)(%) Haze (%) (original) 16000 400000 88.2 3.34 1 roll @ 340 psi 297 69087.3 3.67 1 roll @ 340 psi 108 230 87.2 4.13 1 roll @ 340 psi 73 12786.6 4.19 1 roll @ 340 psi 61 92 87.1 4.47 1 roll @ 340 psi 53 86 86.64.44 Ar plasma 38 62 88.0 4.19

Prior to any application of pressure, the network layers had theresistance indicated in the “original” row (the network layers were notpre-treated with plasma.) Each row of Table 2 indicates a subsequentsingle roll across the network layer at approximately 340 psi.

In each trial, the network layer was rolled 5 times. Thereafter, aplasma treatment was applied to the network layer. The resistance aftereach roll is as listed in the second (first trial) and third (secondtrial) columns. Variation in transmission and haze for the second trialis as listed in the fourth and fifth columns, respectively. As shown, itwas determined that the conductivity of the network layer of each trialwas increased by application of pressure to a surface thereof.

As shown above in Table 2, application of pressure to a network layer bya roller can reduce the light transmission of the layer and increase thehaze. As shown in Table 3 below, a washing process following thepressure treatment can further improve the transmission and reduce thehaze of the network layer.

TABLE 3 Process Resistance (Ω/□) Transmission (%) Haze (%) (original)700,000 86.4 4.77 2 rolls @ 340 psi 109 85.6 5.24 soap & water wash 4486.0 4.94 Ar plasma 24 85.9 4.81

As shown in Table 3, application of pressure to a network layer by twicerolling with a single stainless steel bar at approximately 340 psi on arigid surface reduced the light transmission and increased the haze ofthe network layer. Washing the network layer with soap and water afterthe rolling, however, increased the transmission and decreased the haze.An argon plasma treatment further improved the transmission and haze.

Washing the network with soap and water without rolling is alsoeffective at improving the conductivity to some extent.

Following the pressure or washing treatments, a matrix material can becoated as previously described in Example 2.

Example 6 Photo-Patterning of Conductive Layers

FIG. 25A illustrates one method of directly patterning a nanowire-basedtransparent conductive film. In this example, a silver nanowire networklayer (“network layer”) 600 was initially formed on a glass substrate604 according to the method described in Example 2. Two holders 610 wereplaced on the glass substrate 604 to define an area 614 for matrixformation. A photo-curable matrix material 618 comprising a mixture ofprepolymers was coated over the network layer 600 within the area 614. Amask 620 was placed upon the holders 610. The mask 620 was a glass slidehaving a number of dark lines 624 of about 500 μm wide. The matrixmaterial was then irradiated under a Dymax 5000 lamp for 90 seconds. Thematrix material cured in the regions exposed to light and remainedliquid in the regions that were masked by the dark lines.

As shown in FIG. 25B, conductive film 630 was obtained after thephoto-patterning described above. The lighter regions 634 were exposedto UV irradiation and the dark regions 638 were masked from the lightexposure. Conductive film 640 was further subjected to an adhesive tapeor a tacky roll to remove the matrix material and the nanowires inuncured regions 644. As shown, the contrast between the uncured regions644 and the cured regions 644 was pronounced. Following the adhesivetape treatment, the concentration of the nanowires dropped below thepercolation threshold in the uncured regions 644. Electricalmeasurements using fine probe tips showed that the uncured regions 644were non-conductive.

FIGS. 26A-F show a photo-patterned conductive layer at highermagnifications. FIG. 26A shows the conductive film 640 immediately afterphoto-curing (5×). FIG. 26B shows the conductive film 640 after theadhesive tape treatment (5×), in which the cured region 648 appears muchlighter than the uncured region 644. At higher magnification (FIGS. 26Cand 26D, 20×), it can be observed that the uncured region 644 has alower concentration of nanowires than the cured region 648. Thiscontrast is more apparent in FIGS. 26E and 26F (100×).

As an alternative to removing the matrix material and nanowires in theuncured region using adhesive tapes or tacky rolls, a solvent may beused to wash the uncured regions. As shown in FIGS. 27A-D, a conductivefilm 700 was prepared as described above and exposed to UV irradiationthrough a brass aperture mask. FIG. 27A shows cured regions (conductiveregions) 710 and uncured regions 720 after being washed with ethanol andwiped. FIGS. 27B-D illustrate, at increasing magnifications, thecontrast of the nanowire concentration in the uncured regions 720compared to that in the cured regions 710. In the uncured regions 720,most of the uncured matrix material and the silver nanowires had beenremoved by the ethanol washing. Photo-patterning therefore producesconductive regions and non-conductive region according to apredetermined pattern.

Example 7 Photo-Curable Formulations

The matrix material described in Example 6 can be formulated bycombining an acrylate monomer (or prepolymer, as defined herein), amulti-functional acrylate monomer (or prepolymer) and at least onephotoinitiator. Any acrylate monomers or prepolymers can be used, suchas epoxy acrylates, more specifically, 2-ethylhexyl acrylate,2-phenoxyethyl acrylate, lauryl acrylate, methacrylates, and the like.Any multi-functional acrylate monomer (or prepolymer) can be used topromote the formation of a crosslinking polymer network. Examplesinclude trimethylolpropane triacrylate (TMPTA), tripropylene glycoldiacrylate, bisphenol-A diacrylate, propoxylated (3) trimethylolpropanetriacrylate, dipentaerythritol penta-acrylate. Any photoinitiator, forexample, ketone based initiators, can be used. Specific examplesinclude: Ciba Irgacure 754, phenyl ketone such as Ciba Irgacure 184,α-hydroxy ketones, glyoxylates, benzophenone, α-amino ketones and thelike. More specifically, a fast-curing formulation can be prepared bycombining 60%-70% 2-ethylhexyl acrylate, 15%-30% trimethylolpropanetriacrylate and about 5% Ciba Irgacure 754.

Other additives can be added to enhance the stability and/or promote theadhesion of the matrix and the nanowires. For example, an adhesionpromoter (e.g., silanes) that promotes the coupling between organicmatter and inorganic matter can be used. Examples of the silane-typeadhesion promoters include GE Silquest A174, GE Silquest A1100 and thelike. Antioxidants such as Ciba Irgonox 1010ff, Ciba Irgonox 245,Irgonox 1035 can be used. Moreover, additional or co-initiators can beused to promote the efficiency of the photoinitiator. Examples ofcoinitiator can include any types of tertiary amine acrylates, such asSartomer CN373, CN 371, CN384, CN386 and the like. An additionalphotoinitiator such as Ciba Irgacure OXE01 can be further added.

Below are four exemplary photo-curable formulations suitable as thematrix material used in this example:

Formulation 1 75% 2-ethylhexyl acrylate; 20% trimethylolpropanetriacrylate (TMPTA); 1% adhesion promoter (GE Silquest A1100); 0.1%antioxidant (Ciba Irgonox 1010ff) and 4% photoinitiator (Ciba Irgacure754) Formulation 2 73.9% 2-ethylhexyl acrylate; 20% trimethylolpropanetriacrylate (TMPTA); 1% adhesion promoter (GE Silquest A1100); 0.05%antioxidant (Ciba Irgonox 1010ff) and 5% photoinitiator (Ciba Irgacure754) Formulation 3 73.1% tripropylene glycol diacrylate (TPGDA) 22.0%trimethylolpropane triacrylate (TMPTA) 4.9% photoinitiator (CibaIrgacure 754) 0.03% antioxidant (4-methoxyphenol) Formulation 4 68%2-ethylhexyl acrylate; 20% trimethylolpropane triacrylate (TMPTA); 1%adhesion promoter (GE Silquest A1100); 0.1% antioxydant (Ciba Irgonox1010ff) and 5% photoinitiator I (Ciba Irgacure 754) 5% coinitiator(Sartomer CN373) 1% photoinitiator II (Ciba Irgacure OXE01)

Example 8 Nanowire Dispersion

A nanowire dispersion, or ink, was formulated by combining about 0.08%wt. HPMC, about 0.36% wt. silver nanowires, about 0.005% wt. Zonyl®FSO-100 and about 99.555% wt. water. As an initial step, an HPMC stocksolution was prepared. An amount of water equal to about ⅜ of the totaldesired volume of nanowire dispersion was placed in a beaker and heatedto between 80° C. and 85° C. on a hotplate. Enough HPMC to make 0.5% wt.HPMC solution was added to the water and the hotplate was turned off.The HPMC and water mixture was stirred to disperse the HPMC. Theremainder of the total amount of water was chilled on ice and then addedto the heated HPMC solution and stirred at high RPM for about 20 min.The HPMC solution was filtered through a 40 μm/70 μm (absolute/nominal)Cuno Betapure filter to remove undisolved gels and particulates. Next astock solution of Zonyl® FSO-100 was prepared. More specifically, 10 gof Zonyl® FSO 100 were added to 92.61 mL of water and heated until theZonyl® FSO 100 was fully dissolved. The necessary amount of HPMC stocksolution to make about 0.08% wt. HPMC solution in the final inkcomposition was placed in a container. Then, the necessary amount of Diwater to make about 99.555% wt. water solution in the final inkcomposition was added. The solution was stirred for about 15 min. andthe necessary amount of silver nanowires to make about 0.36% Ag nanowiresolution in the final ink composition were added. Finally, the necessaryamount of the Zonyl® FSO-100 stock solution to make about 0.005% wt.Zonyl® FSO-100 solution was added.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

The invention claimed is:
 1. A method of fabricating a transparentconductor comprising: depositing a coating composition on a surface of asubstrate, the coating composition comprising: a liquid and a pluralityof conductive nanostructures consisting of silver nanowires, wherein thesilver nanowires are 0.05% to 1.4% of a total weight of the coatingcomposition; forming a silver nanowire network layer on the substrate byallowing the liquid to dry, wherein the silver nanowires intersect toprovide the silver nanowire network layer; providing a corrosioninhibitor; forming a matrix material layer on the silver nanowirenetwork layer, wherein the matrix material layer is conductive; anddefining a pattern with a first region and a second region, wherein thesilver nanowire network layer is located in the first region to form aconductive region, the matrix material layer in the second region istotally removed and the silver nanowires in the second region arepartially removed to reduce a concentration of the silver nanowires inthe second region to a level below a percolation threshold so that thesecond region defines a non-conductive region.
 2. The method of claim 1wherein the coating composition further comprises an additive selectedfrom carboxy methyl cellulose, 2-hydroxy ethyl cellulose, hydroxy propylmethyl cellulose, methyl cellulose, poly vinyl alcohol, tripropylenegylcol, and xanthan gum.
 3. The method of claim 1 further comprisingpre-treating the surface of the substrate prior to depositing thecoating composition.
 4. The method of claim 3 wherein pre-treating thesurface of the substrate creates a pattern comprising at least onepre-treated region and at least one untreated region.
 5. The method ofclaim 4 wherein the silver nanowire network layer is only formed on thepre-treated region.
 6. The method of claim 3 wherein pre-treating thesurface of the substrate comprises depositing an intermediate layer onthe surface of the substrate, performing a plasma treatment to thesurface of the substrate, performing an UV-ozone treatment to thesurface of the substrate, or performing a corona discharge treatment tothe surface of the substrate.
 7. The method of claim 1 furthercomprising applying pressure, heat or a combination thereof to thesilver nanowire network layer.
 8. The method of claim 1 wherein formingthe matrix material layer on the silver nanowire network layercomprises: depositing a matrix material on the silver nanowire networklayer; and curing the matrix material to form the matrix material layer,wherein the silver nanowires are dispersed or embedded in the matrixmaterial layer to form a conductive layer.
 9. The method of claim 8further comprising: causing at least a section of each of the silvernanowires to protrude above a surface of the matrix material layer toprovide a conducting surface of the conductive layer.
 10. The method ofclaim 8 wherein the matrix material comprises a polymer dispersed in asolvent or a prepolymer.
 11. The method of claim 10 wherein theprepolymer is photo-curable.
 12. The method of claim 8 wherein thematrix material layer is formed into a patterned matrix material layer.13. The method of claim 8 wherein the substrate is flexible.
 14. Themethod of claim 13 wherein curing the matrix material comprisescontinuously exposing the matrix material to light irradiation, and thelight irradiation is projected to the matrix material according to apattern.
 15. The method of claim 8 wherein the substrate is a flexibledonor substrate.
 16. The method of claim 15 wherein the flexible donorsubstrate is coated with a release layer.
 17. The method of claim 15further comprising detaching the conductive layer from the flexibledonor substrate and applying the conductive layer to a substrate ofchoice.
 18. The method of claim 1 wherein each of the silver nanowireshas an aspect ratio of about 50 or more.
 19. The method of claim 1wherein each of the silver nanowires has an aspect ratio of about 100 ormore.
 20. The method of claim 1 wherein the liquid is water, an alcohol,a ketone, an ether, a hydrocarbon or an aromatic solvent.
 21. The methodof claim 1 wherein the coating composition further comprises one or moresurfactants.
 22. The method of claim 21 wherein the one or moresurfactants are fluorinated alkyl ester, polyoxyethylene octyl phenylether, 2,5,8,11-tetraethyl-6-dodecyn-5,8-diol ethoxylate, orn-Dodecyl-β-D-maltoside.
 23. The method of claim 1 wherein a portion ofthe silver nanowire network layer extending through the matrix materiallayer has a threshold surface loading level of about 0.05 μg/cm² toabout 10 μg/cm².
 24. The method of claim 23 wherein the thresholdsurface loading level is about 0.1 μg/cm² to about 5 μg/cm².
 25. Themethod of claim 24 wherein the threshold surface loading level is about0.8 μg/cm² to about 3 μg/cm².
 26. The method of claim 1 wherein thematrix material layer comprises a conductive polymer.
 27. The method ofclaim 26 wherein the conductive polymer comprisespoly(3,4-ethylenedioxythiophene), polyanilines, polythiophenes, orpolydiacetylenes.
 28. The method of claim 1 wherein the silver nanowiresare embedded in a portion of the matrix material layer.
 29. The methodof claim 1 wherein the silver nanowires are alloyed or plated withanother metal.
 30. The method of claim 29 wherein the silver nanowiresare plated with gold.
 31. The method of claim 1 wherein the corrosioninhibitor binds to the silver nanowires to form a protective film on thesilver nanowire network layer.
 32. The method of claim 1 wherein thesilver nanowire network layer further comprises a plurality ofreservoirs each containing the corrosion inhibitor.
 33. The method ofclaim 1 wherein the liquid comprises solvents that are 94.5% to 99.0% ofthe total weight of the coating composition; and wherein the coatingcomposition further comprises: a surfactant that is 0.0025% to 0.1% ofthe total weight of the coating composition; and a viscosity modifierthat is 0.02% to 4% of the total weight of the coating composition. 34.The method of claim 1 wherein providing the corrosion inhibitorcomprises dipping the silver nanowire network layer in a solution of thecorrosion inhibitor.
 35. The method of claim 1 wherein providing thecorrosion inhibitor comprises coating a layer of the corrosion inhibitoron the silver nanowire network layer.
 36. The method of claim 1 whereinproviding the corrosion inhibitor comprises incorporating the corrosioninhibitor in the matrix material layer.
 37. The method of claim 1wherein forming the matrix material layer on the silver nanowire networklayer comprises: coating a prepolymer on the silver nanowire networklayer; curing the prepolymer in the first region; and totally removingthe prepolymer in the second region.
 38. The method of claim 1 furthercomprising: exposing the silver nanowire network layer in a reducingagent.
 39. The method of claim 38 wherein the reducing agent comprisesborohydrides, boron nitrogen compounds, or hydrogen gas.